VLT (Very Large Telescope) of ESO on Cerro Paranal
VLT is a telescope facility operated by ESO (European Southern Observatory) on the Cerro Paranal mountain in the Atacama Desert of northern Chile at an elevation of 2,635 m (coordinates: 24°37'38''S, 70°24'17''W).
VLT is the flagship facility for European ground-based astronomy at the beginning of the third Millennium. It is the world's most advanced optical instrument, consisting of four Unit Telescopes with main mirrors of 8.2m diameter and four movable 1.8m diameter ATs (Auxiliary Telescopes). The telescopes can work together, to form a giant ‘interferometer', the ESO VLTI (Very Large Telescope Interferometer), allowing astronomers to see details up to 25 times finer than with the individual telescopes. The VLTI functions like a telescope with a mirror 200 m in diameter. The light beams are combined in the VLTI using a complex system of mirrors in underground tunnels where the light paths must be kept equal to distances less than 1/1000 mm over a hundred meters. With this kind of precision, the VLTI can reconstruct images with an angular resolution of milliarcseconds (marcsec), equivalent to distinguishing the two headlights of a car at the distance of the Moon. 1)
ESO is the foremost intergovernmental astronomy organization in Europe and the world's most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile and by Australia as a strategic partner. ESO carries out an ambitious program focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organizing cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world's most advanced visible-light astronomical observatory and two survey telescopes. VISTA (Visible and Infrared Survey Telescope for Astronomy) works in the infrared and is the world's largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-meter ELT (Extremely Large Telescope), which will become "the world's biggest eye on the sky".
Figure 1: Aerial view of the observing platform on the top of Cerro Paranal, with the four enclosures for the 8.2-m UTs (Unit Telescopes) and various installations for the VLT Interferometer (VLTI). Three 1.8 m VLTI ATs (Auxiliary Telescopes) and paths of the light beams have been superimposed on the photo. Also seen are some of the 30 "stations" where the ATs will be positioned for observations and from where the light beams from the telescopes can enter the Interferometric Tunnel below. The straight structures are supports for the rails on which the telescopes can move from one station to another. The Interferometric Laboratory (partly subterranean) is at the center of the platform (image credit: ESO)
The 8.2m diameter Unit Telescopes can also be used individually. With one such telescope, images of celestial objects as faint as magnitude 30 can be obtained in a one-hour exposure. This corresponds to seeing objects that are four billion (four thousand million) times fainter than what can be seen with the unaided eye.
The large telescopes are named Antu, Kueyen, Melipal and Yepun, which are the names for the Sun, the Moon, the Southern Cross, and Venus in the language of the Mapuche people.
Figure 2: Alternate view of ESO's Paranal Observatory hosting several world-class telescopes; among them are the Very Large Telescope, the Visible and Infrared Survey Telescope for Astronomy, and the VLT Survey Telescope. Other scientific and support facilities are also located at Paranal, including several smaller telescopes and an innovative accommodation facility known as the Residencia (image credit: ESO) 2)
ESO (European Southern Observatory) is a 16-nation intergovernmental research organization for ground-based astronomy. Created in 1962, ESO has provided astronomers with state-of-the-art research facilities and access to the southern sky. The organization employs about 730 staff members and receives annual member state contributions of approximately €131 million. Its observatories are located in northern Chile. 3)
ESO has built and operated some of the largest and most technologically advanced telescopes. These include the NTT (New Technology Telescope), an early pioneer in the use of active optics, and the VLT (Very Large Telescope), which consists of four individual telescopes, each with a primary mirror 8.2 m across, and four smaller auxiliary telescopes. The ALMA (Atacama Large Millimeter Array) observes the universe in the millimeter and su-millimeter wavelength ranges, and is the world's largest ground-based astronomy project to date. It was completed in March 2013 in an international collaboration by Europe (represented by ESO), North America, East Asia and Chile.
Currently under construction is the ELT (Extremely Large Telescope). It will use a 39.3 meter diameter segmented mirror, and become the world's largest optical reflecting telescope when operational in 2024. Its light-gathering power will allow detailed studies of planets around other stars, the first objects in the universe, supermassive black holes, and the nature and distribution of the dark matter and dark energy which dominate the universe.
ESO's observing facilities have made astronomical discoveries and produced several astronomical catalogs. Its findings include the discovery of the most distant gamma-ray burst and evidence for a black hole at the center of the Milky Way.
In 2004, the VLT allowed astronomers to obtain the first picture of an extrasolar planet (2M1207b) orbiting a brown dwarf 173 light-years away. The HARPS (High Accuracy Radial Velocity Planet Searcher) instrument installed in another ESO telescope led to the discovery of extrasolar planets, including Gliese 581c—one of the smallest planets seen outside the solar system.
Construction of the VLT began in 1991, and its first observations were made in 1998. Among the VLT's notable discoveries are the first direct spectrum of an extrasolar planet, HR 8799c, and the first direct measurement of the mass of an extrasolar planet, HD 209458b. The VLT also discovered the most massive star known, R136a1, which has a mass 265 times that of the Sun. The VLT is operated by the European Southern Observatory. 4)
Chilean observation sites:
Although ESO is headquartered in Garching, Germany, its telescopes and observatories are in northern Chile, where the organization operates advanced ground-based astronomical facilities:
• La Silla, which hosts the New Technology Telescope (NTT)
• Paranal, where the VLT (Very Large Telescope) is located
These are among the best locations for astronomical observations in the southern hemisphere. An ESO project is the ELT (Extremely Large Telescope), a 40 m class telescope based on a five-mirror design and the formerly planned Overwhelmingly Large Telescope. The ELT will be the largest optical near-infrared telescope in the world. ESO began its design in early 2006, and aimed to begin construction in 2012. Construction work at the ELT site started in June 2014. As decided by the ESO council on 26 April 2010, a fourth site (Cerro Armazones) is to be home to ELT.
Each year about 2,000 requests are made for the use of ESO telescopes, for four to six times more nights than are available. Observations made with these instruments appear in a number of peer-reviewed publications annually; in 2009, more than 650 reviewed papers based on ESO data were published.
ESO telescopes generate large amounts of data at a high rate, which are stored in a permanent archive facility at ESO headquarters. The archive contains more than 1.5 million images (or spectra) with a total volume of about 65 TB of data.
Table 1: ESO telescopes
Figure 3: Aerial view of Paranal with VISTA in the foreground and the VLT (Very Large Telescope) in the background (image credit: ESO/G.Hüdepohl)
Some VLT development states:
• On 17 March 2011 light collected by all four of the 8-meter Unit Telescopes of ESO's Very Large Telescope was successfully combined for the first time using PIONIER (a visiting instrument at the Paranal Observatory, developed at LAOG/IPAG in Grenoble, France), a new generation instrument in the VLT Interferometer. 5)
- To have all four Unit Telescopes (UTs) finally working together as a single telescope is a major step in the development of the VLT — the original design always anticipated that the four 8 m telescopes would be able to work either independently or together as part of the giant VLT Interferometer (VLTI). Coincidentally, the new observations took place on the 10th anniversary of the first successful combination of two beams within the VLTI.
- Among the main science goals for the four UTs, working together with PIONIER, are to try to reveal the signatures of planets in the making, to explore the natures and fates of stars by providing images of their surfaces and their environments and to understand better the powerful engines associated with black holes at the centers of galaxies.
- When combined, the UTs can potentially provide image sharpness that equals that of a telescope with a diameter of up to 130 meters. Three telescopes have been combined regularly since the VLT/VLTI began observing, offering three unique baselines; the ability to combine four telescopes bumps this number up to six and allows the very fine structure in astronomical objects to be studied much more easily.
- Individually the 8meter telescopes can spy objects four billion times fainter than the naked eye can see and working together the four large telescopes can pick up details about 16 times finer than can be seen with one UT.
- An earlier important step towards unleashing the full potential of the VLTI was when light from all four of the 1.8 meter VLT Auxiliary Telescopes (ATs) was combined using PIONIER.
Figure 4: Photo of the VLT platform in 2011 (image credit: ESO)
• November 4, 2010: Light coming from the four 1.8 meter Auxiliary Telescopes at the European Southern Observatory's VLTI (Very Large Telescope Interferometer) based in Paranal, Chile, has been successfully combined for the first time using a new visiting instrument called PIONIER (France). This is an important step towards unleashing the full potential of the VLTI to use multiple telescopes together to reveal fine detail in distant objects. A joint team from Grenoble LAOG (Laboratoire d'Astrophysique de Grenoble) and ESO achieved this very challenging feat of engineering only five days after unpacking the equipment on the mountain. 6)
- The VLTI engineers had to control the distance traversed by the light from the widely separated telescopes with an accuracy of about one hundredth of the thickness of a strand of human hair. Once the light reached PIONIER, it was then channelled into the heart of the instrument: a remarkable optical circuit, smaller than a credit card, that finally brought the light waves from the different telescopes together in a very precise way so that they could create interference. The resulting resolving power of the telescope array has the sharpness not of the individual 1.8 meter Auxiliary Telescopes, but that of a much bigger "virtual telescope" about 100 m across, limited only by how far apart the telescopes can be positioned.
- PIONIER, developed at LAOG in Grenoble, France, is a visiting instrument at the Paranal Observatory, complementing ESO's existing AMBER and MIDI instruments. AMBER has previously combined the light from three of the telescopes at the VLTI to study many objects, including the surface of the variable star T Leporis (eso0906). PIONIER, however, will eventually allow the VLTI to go one stage further; with the additional information that a fourth telescope brings to the table, it should be possible to use complex mathematical processing techniques to create more detailed images. The PIONIER team hopes to produce its first images by early 2011.
Figure 5: Photo of the PIONIER instrument, shown here in the VLTI laboratory at ESO's Paranal Observatory in Chile, has been used to combine the light from the four 1.8 meter Auxiliary Telescopes for the first time (image credit: ESO)
ESPRESSO (Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations)
ESPRESSO is a super-stable Optical High Resolution Spectrograph for the combined coude' focus of the VLT. It can be operated by either one of the UTs or collecting the light from up to 4 UTs simultaneously.
The main scientific drivers for ESPRESSO are:
• the measurement of high precision radial velocities of solar type stars for search for rocky planets
• the measurement of the variation of the physical constants
• the analysis of the chemical composition of stars in nearby galaxies
These science cases require an efficient, high-resolution, extremely stable and accurate spectrograph.
ESPRESSO, installed at the VLT facility, will combine unprecedented radial velocity precision with the large collecting area of the UTs. Moreover it will be capable of collecting the light simultaneously from the 4UTs, to measure precisely faint or high redshift objects.
The understanding of the formation and evolution of planetary systems is one of the most exciting science cases of these days. The radial velocity technique has been so far the most productive in terms of extra-solar planet detections. Low mass planets (one to few Earth masses) are especially interesting because according to formation models they could represent the bulk of the planet population. However they are more elusive and require extremely stable instruments. The HARPS instrument, with a precision better than 1m/s, has discovered up to now the vast majority of planets with masses smaller than Neptune, giving an invaluable experience in view of the realization of more precise instruments. With a radial velocity precision better than 10 cm/s, an Earth mass planet in the habitable zone of a low mass star can be detected.
The instrument will also have the capability to acquire the most accurate measurements of the fundamental constants α (the fine structure constant) and µ (the proton to electron mass ratio) as a function of redshift, therefore addressing the question of whether the constants of the Standard Model of Physics vary with the age of the Universe. Its capability to collect the light from all the 4 UTs simultaneously will enlarge the number of accessible QSOs (Quasi-Stellar Objects) with great benefit for this science case.
The combination of high resolution and high efficiency opens the possibility of measuring the chemical composition of stars in galaxies other than the Milky Way with unprecedented accuracy.
Given the large light collecting power and its efficiency, its high spectral resolution, and its extreme radial velocity precision and accuracy, we expect that ESPRESSO will not only fulfill its main scientific objectives, but also open new opportunities in observational Astronomy with hopefully new and unexpected results.
ESPRESSO was designed and built by a consortium consisting of: the Astronomical Observatory of the University of Geneva and University of Bern, Switzerland; INAF (Osservatorio Astronomico di Trieste) and INAF (Osservatorio Astronomico di Brera), Italy; Instituto de Astrofísica de Canarias, Spain; Instituto de Astrofisica e Ciências do Espaço, Universidade do Porto and Universidade de Lisboa, Portugal; and ESO. The co-principal investigators are Francesco Pepe (University of Geneva, Switzerland), Stefano Cristiani (INAF (Osservatorio Astronomico di Trieste), Italy, Rafael Rebolo (IAC, Tenerife, Spain) and Nuno Santos (Instituto de Astrofisica e Ciencias do Espaco, Universidade do Porto, Portugal).
ESPRESSO is a fiber-fed, cross-dispersed, high-resolution, Echelle spectrograph. The telescope light is fed to the instrument via a Coude-Train optical system and fibers. ESPRESSO is located in the VLT CCL (Combined-Coude Laboratory) at the incoherent focus, where a front-end unit can combine the light from up to 4 Unit Telescopes (UT) of the VLT. The telescope light is fed to the instrument via a so-called Coudé Train optical system and within optical fibers. The target and sky light enter the instrument simultaneously through two separate fibers, which together form the slit of the spectrograph. 7) 8)
The incoherent combined focus: a new facility for a larger telescope: Although foreseen since 1977 in the original VLT plan, the incoherent combined focus of the VLT has never been implemented. Only provision for it, in terms of space left in the UTs structures and ducts in the rock of the mountain, is what is actually available at VLT. As part of the project agreement, the ESPRESSO Consortium has been asked to materialize such a focus providing the necessary hardware and software as part of the deliverables. The implementation of the Coudé Train is requiring substantial changes in the Paranal Observatory infrastructure yielding to an elaborated interfaces management. This new facility will allow to use the four telescopes as a large 16 meter equivalent telescope.
Enhanced flexibility and power: ESPRESSO will be located in the VLT's CCL and, unlike any other instrument built so far, will receive light from any of the four UTs, allowing for a more flexible usage of the observation time. The light of the single UT scheduled to work with ESPRESSO is then fed into the spectrograph (single-UT modes). Alternatively, the combined light of all the UTs can be fed into ESPRESSO simultaneously (multi-UT mode).
Figure 6: The ESPRESSO optical design (image credit: ESPRESSO Consortium)
Several optical ‘tricks' have been used to obtain high spectral resolution and efficiency despite the large size of the telescope and the 1 arcsec sky aperture of the instrument:
• At the spectrograph entrance the APSU (Anamorphic Pupil Slicing Unit) shapes the beam in order to compress it in cross-dispersion and splits in two smaller beams, while superimposing them on the echelle grating to minimize its size. The rectangular white pupil is then re-imaged and compressed.
• Given the wide spectral range, a dichroic beam splitter separates the beam in a blue and a red arm. Each arm is optimized for image quality and optical efficiency.
• The cross-disperser has the function of separating the dispersed spectrum in all its spectral orders. In addition, an anamorphism is re-introduced to make the pupil square and to compress the order height such that the inter-order space and the SNR per pixel are both maximized. Both functions are accomplished using VPHGs (Volume Phase Holographic Gratings) mounted on prisms.
• Finally, two optimized camera lens systems image the full spectrum from 380 nm to 780 nm on two large 92 mm x 92 mm CCDs with 10 µm pixels.
Figure 7: Spectral format and shape of the pseudo slit (image credit: ESPRESSO Consortium)
ESPRESSO development status:
• February 13, 2018: The ESPRESSO instrument on ESO's Very Large Telescope in Chile has used the combined light of all four of the 8.2 m Unit Telescopes for the first time. Combining light from the Unit Telescopes in this way makes the VLT the largest optical telescope in existence in terms of collecting area. This picture shows in highly simplified form how the light collected by all four VLT Unit Telescopes is combined in the ESPRESSO instrument, located under the VLT platform. 9) 10)
- One of the original design goals of ESO's VLT was for its four UTs (Unit Telescopes) to work together to create a single giant telescope. With the first light of the ESPRESSO spectrograph using the four-Unit-Telescope mode of the VLT, this milestone has now been reached.
- After extensive preparations by the ESPRESSO consortium (led by the Astronomical Observatory of the University of Geneva, with the participation of research centers from Italy, Portugal, Spain and Switzerland) and ESO staff, ESO's Director General Xavier Barcons initiated this historic astronomical observation with the push of a button in the control room.
Figure 8: The ESPRESSO instrument achieves first light with all four Unit Telescopes (image credit: ESO, L. Calçada)
• December 6, 2017: ESPRESSO has achieved first light on ESO's Very Large Telescope at the Paranal Observatory in northern Chile. This new, third-generation echelle spectrograph is the successor to ESO's hugely successful HARPS instrument at the La Silla Observatory. HARPS can attain a precision of around 1 m/s in velocity measurements, whereas ESPRESSO aims to achieve a precision of just a few cm/s, due to advances in technology and its placement on a much bigger telescope. 11)
- The lead scientist for ESPRESSO, Francesco Pepe from the University of Geneva in Switzerland, explains its significance: "This success is the result of the work of many people over 10 years. ESPRESSO isn't just the evolution of our previous instruments like HARPS, but it will be transformational, with its higher resolution and higher precision. And unlike earlier instruments it can exploit the VLT's full collecting power — it can be used with all four of the VLT Unit Telescopes at the same time to simulate a 16 m telescope. ESPRESSO will be unsurpassed for at least a decade — now I am just impatient to find our first rocky planet!"
Figure 9: This colorful image shows spectral data from the First Light of the ESPRESSO instrument on ESO's Very Large Telescope in Chile. The light from a star has been dispersed into its component colors. This view has been colorized to indicate how the wavelengths change across the image, but these are not exactly the colors that would be seen visually. Close inspection shows many dark spectral lines in the stellar spectra and also the regular double spots from a calibration light source. The dark gaps are features of how the data is taken, and are not real (image credit: ESO/ESPRESSO team)
- ESPRESSO can detect tiny changes in the spectra of stars as a planet orbits. This radial velocity method works because a planet's gravitational pull influences its host star, causing it to "wobble" slightly. The less massive the planet, the smaller the wobble, and so for rocky and possibly life-bearing exoplanets to be detected, an instrument with very high precision is required. With this method, ESPRESSO will be able to detect some of the lightest planets ever found.
- The test observations included observations of stars and known planetary systems. Comparisons with existing HARPS data showed that ESPRESSO can obtain similar quality data with dramatically less exposure time.
- Although the main goal of ESPRESSO is to push planet hunting to the next level, finding and characterizing less massive planets and their atmospheres, it also has many other applications. ESPRESSO will also be the world's most powerful tool to test whether the physical constants of nature have changed since the Universe was young. Such tiny changes are predicted by some theories of fundamental physics, but have never been convincingly observed.
• September 14, 2015: Engineers at ESO have recently completed the difficult process of aligning the grating. The production and alignment of this component is one of the key ESO contributions to the ESPRESSO project. The grating is the largest ever assembled at ESO, and its length matches the largest echelle grating ever made — the 1.2 x 0.3 m grating for the HIRES spectrograph at the Keck 10 m telescope. 12)
- After its final alignment, the grating is fixed in a permanent mount. All its components are made of Zerodur (the same material that is used for the mirrors of the VLT) and will require no further adjustments, ever. This mounting technique was pioneered at ESO, and demonstrated to work on earlier instruments.
- When installed at ESO's Paranal Observatory in Chile in 2016, ESPRESSO will combine the light from all four Unit Telescopes of the Very Large Telescope to create a virtual 16-meter aperture telescope. Its diffraction grating will split up the light into its component colors for analysis — spreading the light as a prism does, although relying on a different physical mechanism.
Figure 10: The huge diffraction grating at the heart of the ultra-precise ESPRESSO spectrograph — the next generation in exoplanet detection technology — is pictured undergoing testing in the cleanroom at ESO Headquarters in Garching, Germany (image credit: ESO, M. Zamani)
• August 7, 2013: ESO has signed contracts with Winlight Systems (France) for the construction of two cameras for the powerful new exoplanet-finding instrument, ESPRESSO. 13)
- ESPRESSO is an ultra-stable spectrograph that will be installed at ESO's Paranal Observatory in Chile in 2016. It will be capable of combining light from all four Unit Telescopes of the Very Large Telescope (VLT) to create a virtual 16 meter aperture telescope.
- ESPRESSO is expected to allow astronomers to detect Earth-like planets around nearby stars using the radial velocity method . It will also have many other science applications, including the search for possible variations of the constants of nature at different times and in different directions through the study of the light from very distant quasars.
- The new contract is for the provision of the two refractive cameras, one for the red and one for the blue parts of the spectrum. These are vital components of the instrument. Utilizing highly non-spherical surfaces and novel design principles, they achieve excellent image quality over a large field with only three optical elements.
Figure 11: Engineering rendering of the ESPRESSO instrument (image credit: ESO)
HAWK-I (High Acuity Wide field K-band Imager)
HAWK-I is a cryogenic instrument, a wide field K-band imager, which was installed on the adapter/rotator of one of the Nasmyth foci of the VLT Unit Telescope 4 (UT4) as shown in Figures 12 and 13. HAWK-I is equipped with a mosaic of four 2 k x 2 k arrays and operates from 0.85 -2.5 µm over 7.5 arcmin x 7.5 arcmin with 0.1 arcsec pixels. A novel feature is the use of all reflective optics that, together with filters of excellent throughput and detectors of high quantum efficiency, has yielded an extremely high throughput. 14) 15)
Table 2: An overview of the HAWK-I characteristics
With the exception of the entrance window, the optical design is based on an all reflective configuration. The purpose of the optical configuration is to adapt the F-number of the input beam to the pixel field-of-view requirement (0.1 arcsec/pixel) and to limit the stray light reaching the detector by means of a cold field stop located at the entrance of the instrument and the third mirror acting as a cold pupil stop.
Just before the light reaches the detectors, two filter wheels allow the insertion of Broad Band (Y, J, H, Ks) and NarrowBand (interstellar lines and cosmological) filters.
The HAWK-I focal plane is equipped with a mosaic of four 2 k x 2 k Rockwell HgCdTe MBE (Molecular Beam Epitaxy) HAWAII 2 RG arrays. The packaging of this 2 x 2 mosaic detector is provided by GL Scientific re-using the design developed for the JWST program. The acquisition system is based on the IRACE (Infrared Array Control Electronics) system developed at ESO.
Figure 12: The HAWK-I instrument mounted on the telescope's Nasmyth (side) port. HAWK-I is attached on Yepun, Unit Telescope number 4 of ESO's Very Large Telescope and saw First Light on the night of 31 July 2007. HAWK-I covers about 1/10th the area of the Full Moon in a single exposure. It is uniquely suited to the discovery and study of faint objects, such as distant galaxies or small stars and planets (image credit: ESO)
Figure 13: Cut through HAWK-I for an optical and mechanical overview. Blue: optical components; black: cold assembly, filter wheels, detector assembly; green: radiation shield; red: vessel structure, cryogenic components, electronic rack (image credit: ESO)
HAWK-I performance: The 7.5' x 7.5' FOV of HAWK-I is covered by four Hawaii-2RG chips of 2048 x 2028 pixels each (1 pixel corresponds
Finally, due to necessary baffling in the all-reflective optical design of HAWK-I, some vignetting at the edges of the field has turned out to be inevitable due to positioning tolerances of the light baffles. The vignetting measured on sky is summarized in Table 3. Note that although the +Y edge vignetting is small in amplitude, it extends to around 40 pixels at <10%.
Figure 14: HAWK-I field-of-view coverage by the detector mosaic. Left: the layout of the field of view on the sky – note the small gap of ~15'' between the four detectors. Right: the relative orientation of the four detectors with the average gap size given in pixels (image credit: ESO)
Table 3: HAWK-I field-of-view vignetting. Note: The last column represents the maximum extinction of a vignetted pixel, i.e. the percentage of light absorbed in the pixel row or column, with respect to the mean of the field.
GRAAL (Ground-layer AOM (Adaptive Optics Module) Assisted by Lasers):
HAWK-I is also designed to work in the future with a GRAAL as part of the AOF (Adaptive Optics Facility) for VLT. GRAAL) will improve the quality of HAWK-I images reducing by 12 % in Y and 21% in Ks band the diameter collecting 50% of the energy for 1 arcsec visible seeing conditions over the entire 7.5 arcmin x 7.5 arcmin FOV. 16)
The commissioning instrument for the DSM is a submodule of GRAAL labeled MCM (Maintenance & Commissioning Mode) and will be implemented inside the AO module. One 40 x 40 SH (Shack-Hartmann) retractable WFS (Wave Front Sensor) and its associated optical system will be mounted on the AO structure and will use NGS (Natural Guide-Star) on-axis for wavefront sensing. The AO module consists of the following units (Figure 15):
Figure 15: Mechanical design of GRAAL. Left: view from the Hawk-I side; Right: view from the Nasmyth flange side (image credit: ESO)
As of 2017, GRAAL is part of AOF and associated with the DSM (Deformable Secondary Mirror) and the 4 Laser Guide Star Facility (4LGSF). It defines an AO system developed to increase the performance of the HAWK-I instrument. Commissioning is proceeding and Science Verification is planned for January 2018, for which applications are invited. 17)
GALACSI (Ground Atmospheric Layer Adaptive Optics for Spectroscopic Imaging):
GALACSI is part of the AOF (Adaptive Optics Facility), and associated with the DSM and the 4LGSF it defines an AO system developed to increase the performance of the MUSE (Multi Unit Spectroscopic Explorer) instrument, a panoramic integral-field spectrograph working at visible wavelength built by a consortium led by CRAL (Centre de Recherche Astrophysique de Lyon).The system GALACSI combined with the MUSE instrument is dubbed MUSE facility.
GALACSI and GRAAL benefit from several common developments for the AOF and other applications. Specifically these are the DSM and the ASSIST (Adaptive Secondary Setup and Instrument Simulator (AOF test bench)), 4-LGSF, NGC (New General detector Controller) WFS camera and the SPARTA (Real Time Computer platform for the AOF).
FLAMES (Fiber Large Array Multi Element Spectrograph)
LAMES is the multi-object, intermediate and high resolution spectrograph of the VLT. Mounted at the Nasmyth A platform of UT2,FLAMES can access targets over a large corrected field of view (25 arcmin diameter). It consists of three main components: 18)
• A Fiber Positioner (OzPoz) hosting two plates: while one plate is observing the other positions the fibers for the subsequent observations, therefore limiting the dead time between one observation and the next to less than 15 minutes, including the telescope preset and the acquisition of the next field.
• A medium-high resolution optical spectrograph, GIRAFFE (Fiber-fed multi-object spectrograph and part of the VLT) FLAMES facility), with three types of feeding fiber systems : MEDUSA, IFU, ARGUS.
• A link to the UVES (V-Visual Echelle Spectrograph) instrument (Red Arm) via 8 single fibers of 1 arcsec entrance aperture.
Special observing software (FLAMES OS) coordinates the operation of the different subsystems, also allowing simultaneous acquisition of UVES and GIRAFFE observations. For combined observations, the exposure times for UVES and GIRAFFE do not need to be the same. Note that it is not possible to observe simultaneously in two GIRAFFE modes, or to observe the same target simultaneously with the two spectrographs.
• GIRAFFE is a medium-high (R=5500-65100) resolution spectrograph for the entire visible range, 370-950 nm. It is equipped with two gratings and several filters are available to select the required spectral range. Five additional fibers allow simultaneous wavelength calibration of every exposure. Each object can be only observed in one, or a fraction of a single echelle order at once. The fiber system feeding GIRAFFE consists of the following components:
- The MEDUSA fibers, which allow up to 132 separate objects(including sky fibers) to be observed in one go. Two separate sets of MEDUSA fibers exists, one per positioner plate. Each fiber has an aperture of 1.2 arcsec on the sky.
- The IFU (Integral Field Unit): each deployable IFU consists of a rectangular array of 20 microlenses of 0.52 arcsec each,giving an aperture of 2 x 3 arcsecs. For each plate there are 15 IFU units dedicated to objects and another 15 dedicated to sky measurements. In the latter, only the central fiber is present.
- ARGUS: the large integral field unit ARGUS is mounted at the center of one plate of the fiber positioner and consists of a rectangular array of 22 x 14 microlenses. Two magnification scales are available: ``1:1'' with a sampling of 0.52 arcsec/microlens and a total aperture of 11.5 x 7.3 arcsec, and ``1:1.67'' with 0.3 arcsec/microlens and a total aperture of 6.6 x 4.2 arcsec. In addition, 15 ARGUS sky fibers can be positioned in the 25 arcmin field.
GIRAFFE is equipped with one 2 x 4K EEV CCD (15 µm pixels),with a scale of 0.3 arcsec/pixel in MEDUSA, IFUs and ARGUS direct mode,and a scale of 0.15 arcsec/pixel in the enlarged ARGUS mode. GIRAFFE is operated with 39 fixed setups (31 high resolution + 8 low resolution modes).
• UVES is the high resolution spectrograph at UT2 of the VLT (see Section6.4). It was designed to work in long slit mode but it has been possible to add a fiber mode (6 to 8 fibers, depending on setup and/or mode) fed by the FLAMES positioner to its Red Arm only. Only the three standard UVES Red setups are offered, with central wavelength of 520,580 and 860 nm, respectively.
The standard readout mode of FLAMES-UVES is 225 kHz (unbinned) which ensures low readout noise. As of P76 a high-speed readout mode (625 kHz, unbinned, low gain) with increased readout noise but less overheads is offered in visitor mode only.
With an aperture on the sky of 1 arcsec, the fibers project onto 5 UVES pixels giving a resolving power of 47000. For faint objects and depending on the spectral region, one or more fibers can be devoted to recording the sky contribution. In addition, for the 580 nm setup only, a separate calibration fiber is available to acquire simultaneous ThAr (Thorium Argon) calibration spectra. This allows very accurate radial velocity determinations. In this configuration, 7 fibers remain available for targets on sky.
FORS2 (FOcal Reducer/low dispersion Spectrograph 2)
FORS2 is a multi mode (imaging, polarimetry, long slit and multi-object spectroscopy) optical instrument mounted on the UT1 Cassegrain focus of VLT. FORS2 works in the wavelength range 330-1100 nm. Two different magnifications can be used with pixel scales of 0.25''/pixel (with the Standard Resolution collimator) and 0.125''/pixel (with the High Resolution collimator). The corresponding field sizes are 6.8' x 6.8' and 4.25' x 4.25', respectively. The two different magnifications are chosen by selecting one of two different collimators, hence each magnification has to be calibrated independently. An unbinned CCD readout mode is only offered for applications that specifically require it and must be explicitly requested in the proposal. 19)
Imaging: FORS2 offers imaging with a wide range of broad- and narrow-band filters. The narrow-band filters are exchangeable and chosen from a large range available filters depending on the user-request. It is also possible to use the jaws of the MOS unit as occulting bars to avoid saturation by unwanted bright objects.
Spectroscopy: FORS2 has a number of grisms available with different resolutions, including a number of high-throughput Volume-Phased Holographic (VPH) grisms.
Long-Slit (LSS) mode: FORS2 has 9 long-slits with fixed widths of between 0.3'' and 2.5''.
Moveable Slitlets (MOS) mode: FORS2 has a set of 19 pairs of arms that can be moved into the focal plane to form slitlets with user-defined widths.
Spectroscopic Mask (MXU) mode: In addition, FORS2 offers the possibility to insert in the focal plane a mask where slits of different length, width and shape can be cut with a dedicated laser cutting machine. Up to 10 masks can be mounted in a mask unit inside the instrument and each mask can have up to 470 slits, depending on the grism and filter used. The FIMS tool must be used for Phase 2 preparation of the mask cutting files. Performance in this mode is equivalent to that of the standard MOS mode.
Polarimetry: The polarimetric modes allow the measurement of linear and circular polarization, both for direct imaging (IPOL) and spectroscopy (PMOS). The position angle and degree of the linear polarization or of the circular polarization of an object are determined by using a remotely controlled rotatable lambda/2- or lambda/4-plate in front of the Wollaston prism.
Note: a field-dependent instrumental polarization pattern was discovered in the FORS1 linear polarization mode. This spurious polarization field shows a high degree of axial symmetry and smoothly increases from less than 3x10e-4 on the optical axis to 7x10e-3 at a distance of 3 arcmin from it (V band). The problem is yet to be characterized on FORS2, but it is likely it will have the same characteristics.
Detector: Two detector systems are available for FORS2. The first is a detector consisting of a mosaic of two 2k x 4k MIT CCD (15 µm pixels), which provides excellent red sensitivity (> 750 nm) and very low fringing. The second is a mosaic of two 2k x 4k E2V CCDs (15 µm pixels), which was formerly the FORS1 (post-upgrade) detector. This is very sensitive in the blue range (< 500 nm) but shows a lot of fringes above 650 nm. The E2V detector is currently only available in Visitor Mode and must be requested at phase I.
Table 4: FORS2 imaging modes
The direct imaging "Mag-limit" is the broad band magnitude calculated for a point source of zero color (A0V star) which would give a S/N of 5 in one hour with dark sky, clear conditions, a seeing FWHM of 0.8'' and an airmass of 1.2. The U, B, V magnitudes are calculated using the broadband filters of the standard instrument configuration.
Table 5: FORS2 spectroscopic modes
Legend to Table 5:  In longslit spectroscopy the slit is chosen out of a set of 9 slits with fixed width between 0.3'' and 2.5''.  In multiobject spectroscopy one may have 19 slitlets of length alternating between 20'' and 22''.
The spectroscopic Mag-limit given in the table above are the R-band magnitudes of a point source of zero color which would give a S/N of5 per pixel at 650 nm (grisms 150 I and 600 R) in the continuum in one hour with dark sky, clear conditions, a seeing FWHM of 0.8'', an airmass of 1.2, and using a 1.0" slit and the SR-collimator. The two limits given are for the two representative resolutions. The limits on spectropolarimetry are those which for linear/circular polarization allow a 1% accuracy in determination of degree of polarization for one hour of total integration time.
FORS Instrumental Mask Simulator (FIMS): To prepare precise target acquisitions at Phase 2, ESO provides the FIMS software tool. FIMS is required when using FORS2 in several spectroscopic modes, and is also used to prepare occulting bar imaging and spectropolarimetry observations. Phase 1 proposers who wish to justify their time request by optimizing movable or MXU slitlet positions during Phase 1, may find it useful to download and install FIMS. Please refer to the FIMS page for instructions on how to install FIMS and to the FIMS User's Manual on how to use FIMS.
Accurate Astrometry or Pre-imaging Required: Highly accurate relative astrometry is required for any observing mode which in Phase 2 will make use of FIMS or blind offset acquisitions. The mask preparation with FIMS requires input images which are astrometrically corrected within the definitions and precision given below. DSS images will, in almost all cases, not be suitable for the task.
In general the relative astrometry must be known to better than 1/6 of the slit widths all over the field of view. Relative astrometry here means that the slit positions must be known relative to those of reference stars in the field of view with the given precision.
If images of adequate quality are not available, Phase 1 proposers must apply for pre-imaging defined as a separate run in the Phase 1 proposal and should be clearly marked as pre-imaging in "instrument configuration" section of the proposal. Failure to do so will, in case the program is approved for execution, result in the deduction of the time necessary for pre-imaging from the allocation destined to the main part of the project. As a rule, pre-imaging runs are carried out in Service Mode, even for programs whose main (spectroscopic) runs are conducted in Visitor Mode.
FOcal Reducer and low dispersion Spectrograph
"Of all instruments at Paranal, this one is the Swiss Army knife". This is the way Henri Boffin, the instrument scientist behind the FOcal Reducer and low dispersion Spectrograph 2 or FORS2, describes the instrument that is most in demand at ESO's Paranal Observatory. The key to success is that FORS2, installed on UT1 (Antu) of the Very Large Telescope (VLT), is able to study many different astronomical objects in many different ways. 20)
For example, FORS2 can take images of relatively large areas of the sky with very high sensitivity. No wonder that some of the most iconic photos taken with the VLT used this instrument (see eso9845d, eso9948f, eso0202a, eso0338a, eso0338c, eso0617a, and more recently eso1244a and eso1348a).
But FORS2 can also take spectra of one (eso9920r), two or even several tens of objects in the sky simultaneously (eso0223b). "When used as a spectrograph, FORS2 disperses the light into very sophisticated rainbows that help astronomers study chemical composition or estimate the distances of remote objects," says Boffin.
And this is not all. FORS2 can also measure the polarization of light and is therefore used at the VLT to determine whether some astronomical objects have strong magnetic fields.
Observations with FORS2 and its twin brother FORS1 (decommissioned in 2009) have together led to almost 1800 papers to be published in scientific journals as of 2014, with an average of about 100 scientific papers per year. "Basically, whatever you can think of, you can do it with FORS2. Apart from making the coffee the astronomers need at night!"
Science highlights with FORS
• Constraining size, shape and color of first-observed interstellar asteroid (eso1737)
• Observations of first light from gravitational wave source (eso1733)
• First detection of titanium oxide in an exoplanet (eso1729)
• Observations of neutron star that possibly confirm 80-year-old prediction about the vacuum (eso1641)
• Observations of galaxy clusters (eso1548)
• Alignments between supermassive black hole axes and large-scale structure revealed (eso1438)
• FORS helps explain shape of planetary nebula (eso1244)
• FORS was used to spot "Dark Galaxies", an early phase of galaxy formation, which are essentially gas-rich galaxies without stars (eso1228)
• VLT "rediscovered" life on Earth (eso1210)
• Comet Halley in the cold – the most distant view of a regular visitor (eso0328)
• Cosmological gamma-ray bursts and hypernovae linked by FORS1 and FORS2 observations (eso0318)
Figure 16: ESOcast 190: Chile Chill 12 — Fire in the Heavens: In Chile Chill 12 the evocative tunes of ESO's Music Ambassador Dimitris Polychroniadis are set to stunning visuals from ESO's expansive video archive. Sit back, relax, and enjoy a stellar walk through the Universe on the border between science and art (ESO, Published on 11January 2019) 21)
Figure 17: This photo shows the twin instruments, FORS2 at KUEYEN (in the foreground) and FORS1 at ANTU, seen in the background through the open ventilation doors in the two telescope enclosures. Although they look alike, the two instruments have specific functions (image credit: ESO) 22)
Figure 18: This intriguing new view of a spectacular stellar nursery IC 2944 is being released to celebrate a milestone: 15 years of ESO's Very Large Telescope. This image also shows a group of thick clouds of dust known as the Thackeray globules silhouetted against the pale pink glowing gas of the nebula. These globules are under fierce bombardment from the ultraviolet radiation from nearby hot young stars. They are both being eroded away and also fragmenting, rather like lumps of butter dropped onto a hot frying pan. It is likely that Thackeray's globules will be destroyed before they can collapse and form new stars (image credit: ESO) 23)
MATISSE (Multi AperTure mid-Infrared SpectroScopic Experiment)
MATISSE is a second-generation interferometry instrument for the ESO VLTI (Very Large Telescope Interferometer). The interconnection of three or four telescopes makes it possible to capture visibilities and closure phases and allows pictures in the mid infrared range to be reconstructed. The maximum baseline length of 200 m (the distance between two telescopes) makes for a high resolution in the generated images. 24) 25) 26)
MATISSE operates in the wavelength range from 2.8 µm to 13 µm. The combined light of the telescopes is spectrally dispersed in order to measure the wavelength dependence of the visibility and closure phase. Two imaging sensors are used to cover the wide wavelength range. One imaging sensor (HAWAII/2RG) covers the wavelength range of the L and M bands (2.8 µm to 5.2 µm), and the other imaging sensor (Aquarius) covers the N band (8 µm to 13 µm). The HAWAII 2RG imaging sensor features low readout noise, while the Aquarius imaging sensor provides a high readout rate. The Aquarius imaging sensor provides precise measurements despite the high thermal background radiation.
- INSU: Observatoire de la Cote d'Azur and University of Sophia-Antipolis, Nice, France
- MPIA (Max-Planck-Institut für Astronomie), Heidelberg, Germany
- MPIfR (Max-Planck-Institut für Radioastronomie), Bonn, Germany
- NOVA (Netherlands Research School for Astronomy), Leiden, The Netherlands
- Institut für Theoretische Physik und Astrophysik, University of Kiel, Germany
- Institut für Astronomie, University of Vienna, Austria.
Scientific objectives: 27)
The objective of MATISSE is image reconstruction. It will extend the astrophysical potential of the VLTI by overcoming the ambiguities existing in the interpretation of simple visibility measurements. MATISSE will measure closure phase relations thus offering an efficient capability for image reconstruction.
The unique performance of MATISSE is partly related to the existence of the four large apertures of the VLT (UTs) that permits to push the sensitivity limits to values required by selected astrophysical programs such as the study of AGN (Active Galactic Nuclei) and protoplanetary discs.
Moreover, the evaluated performance of MATISSE is linked to the availability of ATs (Auxiliary Telescope for the VLTI) which are relocatable in position in about 30 different stations allowing the exploration of the Fourier plane with up to 200 meters baseline length. Key science programs using the ATs cover for example the formation and evolution of planetary systems, the birth of massive stars as well as the observation of the high-contrast environment of hot and evolved stars.
During Phase A, three constituents of the planetary systems were identified for which MATISSE will bring new insight:
• Protoplanetary disks (T Tauri, HAeBe) and planetary debris disks (beta Pic type),
• Minor bodies of our solar system: main belt asteroids and the comets,
• Young giant planets and so-called hot Jupiter-like planets.
The AMBER (Astronomical Multi-BEam combineR (VLTI Instrument)) and MIDI (Mid-infrared Interferometric Instrument (VLTI)) instruments have started to observe the brightest protoplanetay disks in the infrared sky, approximately a dozen objects. The current capabilities of other observatories are similar (see e.g. Keck Interferometer).
In addition, in our own solar system, a few asteroids have been observed by MIDI (Delbo et al. 2009, ApJ 694, 1228), demonstrating the feasibility to characterize solar system minor bodies with interferometry.
However, for extrasolar planet detections and/or characterization, interferometry has not reached enough dynamic range so far to successfully observe any of them (see e.g. Matter et al. 2010, A& A 515, 69 for MIDI, or Millour et al. 2008, SPIE 7013, 41 and Absil et al. 2010 in press for AMBER).
The second important science topic is active galactic nuclei. The nominal MATISSE sensitivity in blind mode at N-band is 0.6 Jy in 4-telescope mode, similar to the nominal MIDI sensitivity, although to date MIDI has been used to obtain correlated fluxes of AGNs down to 0.17 Jy. The nominal L-band sensitivity from the performance analysis is 0.1 Jy. For MIDI observations, various lists of AGNs have been assembled, often based on the list of Veron-City and Veron 2006 (A& A 455, 773).
MATISSE is a mid-infrared spectro-interferometer combining the beams of up to 4 UTs/ATs of the VLT Interferometer. The number of combined beams is 4. The instrument will be able to operate with 3 or 2 beams. The instrument sensitivity, sampling and throughput are optimized for L- and N-band. The L-band is specified from 3.2 to 3.9 µm and the N-band from 8.0 to 13.0 µm. MATISSE will operate also in M-band, from 4.5 to 5.0 µm. The L-, M- and N-bands can be observed simultaneously.
The instrument will be able to observe with different spectral resolutions. 2 spectral resolutions are possible in N-band (R ~ 30, R ~ 200) and 3 in L&M-bands (R ~ 30, R ~ 500 for L- and M-band, R ~1000 for L-band only). Due to readout time, the full simultaneous coverage of the L- & M- bands in low and medium resolutions, and the full coverage of the L- band in high spectral resolution require an external fringe tracker.
MATISSE will measure: coherent flux, visibilities, closure phases and differential phases. Differential visibilities can also be derived. These quantities will be measured as a function of the wavelength in the selected spectral bands and resolutions.
MATISSE will have an imaging mode (2D image observation without dispersion) for field acquisition and a non-interferometric imaging mode (photometric channels) for flux measurements. It will have also internal devices allowing detector calibration (flat-fielding, bad pixel map), relative flux calibration, wavelength calibration, and instrumental contrast measurement.
MATISSE is a four-beam experiment with a multi-axial global combination. The interferometric beam and the photometric beams receive, respectively, the I and P fraction of the incoming flux (observations without photometric channels are also possible). For an observation with 4 telescopes with photometric channels, 5 images are produced on the detector (4 photometric channels and the interferometric one).During observations with 4 telescopes, the interferogram (in each spectral channel) contains a pattern with 6 fringe periods and is dispersed in the spectral direction. The spatial size of this interferometric channel is larger than the photometric ones in order to optimize the sampling of the 6 fringe structures. The beam combination is made by the camera optics. At this level, the beam configuration is non redundant (separation B between beams equal to 3D, 9D and 6D where D is the spatial diameter of the beam) in order to avoid crosstalk between the fringe peaks in the Fourier space.
The Fourier transform of each spectral column of the interferometric image is thus composed by 6 fringe peaks centered at different frequencies Bij/λ (3 D/λ, 6 D/λ, 9 D/λ, 12 D/λ, 15 D/λ, 18 D/λ) and a low frequency peak that contains the object photometry and the thermal background coming from the 4 telescopes. In order to measure the coherent fluxes with a good accuracy, the design of MATISSE is based on the use of spatial filters, including image and pupil stops.
In order to measure closure and differential phases with a good accuracy, a beam commutation can be made in order to reduce the effect of the instrumental defects on the useful signal.
To measure the coherent fluxes and all the derived interferometric measures such as the differential visibility and phase and the closure phase, the key problem is to eliminate the cross talks between the low frequency peak and all the other peaks that introduce sensitivity of the fringe peaks to variations of the thermal background. Two methods are combined in MATISSE to ensure this result with a large margin: spatial modulation like in AMBER combined with temporal modulation like in MIDI. In addition, to measure the absolute visibility we also have to find the true source photometry. To do that, it is necessary to separate the stellar flux from the sky background, using chopping.
Some devices such as artificial sources, hot screen, optics for flat field or pupil visualization, special material for spectral calibration are implemented in the instrument in order to perform alignment, test, maintenance, calibration and acquisition operations. MATISSE will operate with 2 modes:
• High Sensitivity mode: this mode has no photometry and all photons are collected in the interferometric beam. This maximizes the sensitivity and also the SNR on the coherent flux and the differential and closure phases. Chopping is optional in this mode.
• Simultaneous Photometry mode: this mode uses photometry (2/3 of flux in the interferometric channel and 1/3 in the photometric ones) and chopping to measure the average source photometry and therefore extract the visibility from the coherent flux (the chopping period is longer than the coherence time and hence the chopping has no influence on the limiting magnitude). For an observation with 4 telescopes with photometric channels, 5 images are produced on the detector (4 photometric channels and the interferometric one).
MATISSE project development status (Ref. 27):
• Preliminary acceptance at Paranal Observatory, Chile: Scheduled for 2019
• First light on telescope: February 2018
• Preliminary acceptance Europe: September 2017
• Final Design Review, March 2012
• Optical and Cryogenics Final Design Review, September 2011
• Preliminary Design Review, December 2010.
Figure 19: The figure shows a 3D optical layout for the set of four beams inside the cryostat for the L-and M-bands (image credit: MATISSE Consortium)
Figure 20: The very complex instrument MATISSE during installation at Paranal Observatory (image credit: ESO, P. Horálek) 28)
Gravity is a four way beam combination second generation instrument for the VLTI (Very Large Telescope Interferometer ). Its main operation mode makes use of all four 8 m UTs (Unit Telescopes) to measure astrometric distances between objects located within the 2 arcsec FOV (Field of View) of the VLTI. With the sensitivity of the UTs and the ~10 µas (micro arcsec) astrometric precision, it will allow to measure orbital motions within the galactic center with unprecedented precision. Other modes of the instrument will allow imaging and the use of the 1.8 m Auxiliary Telescopes. 29)
GRAVITY was developed by a collaboration consisting of the Max Planck Institute for Extraterrestrial Physics (Germany), LESIA of Paris Observatory–PSL/CNRS/Sorbonne Université/Univ. Paris Diderot and IPAG of Université Grenoble Alpes/CNRS (France), the Max Planck Institute for Astronomy (Germany), the University of Cologne (Germany), the CENTRA–Centro de Astrofísica e Gravitação (Portugal) and ESO. The PI (Principal Investigator) of GRAVITY is Frank Eisenhauer.
GRAVITY is a new instrument to coherently combine the light of the European Southern Observatory Very Large Telescope Interferometer to form a telescope with an equivalent 130 m diameter angular resolution and a collecting area of 200 m2. The instrument comprises fiber fed integrated optics beam combination, high resolution spectroscopy, built-in beam analysis and control, near-infrared wavefront sensing, phase-tracking, dual-beam operation, and laser metrology. GRAVITY opens up to optical/infrared interferometry the techniques of phase referenced imaging and narrow angle astrometry, in many aspects following the concepts of radio interferometry. This article gives an overview of GRAVITY and reports on the performance and the first astronomical observations during commissioning in 2015/16. 30)
Science Objectives: GRAVITY will carry out the ultimate empirical test to show whether or not the Galactic Center harbors a black hole (BH) of four million solar masses and will finally decide if the near-infrared flares from Sgr A* originate from individual hot spots close to the last stable orbit, from statistical fluctuations in the inner accretion zone or from a jet. If the current hot-spot interpretation of the near-infrared (NIR) flares is correct, GRAVITY has the potential to directly determine the spacetime metric around this BH. GRAVITY may even be able to test the theory of general relativity in the presently unexplored strong field limit. GRAVITY will also be able to unambiguously detect intermediate mass BHs, if they exist. It will dynamically measure the masses of supermassive BHs (SMBHs) in many active galactic nuclei, and probe the physics of their mass accretion, outflow and jets with unprecedented resolution. Furthermore, GRAVITY will explore young stellar objects, their circumstellar discs and jets, and measure the properties of binary stars and exoplanet systems. In short, GRAVITY will enable dynamical measurements in an unexplored regime.
GRAVITY provides high precision narrowangle astrometry and phase-referenced interferometric imaging in the astronomical K-band (2.2 µm). It combines the light from four Unit Telescopes (UTs) or Auxiliary Telescopes (ATs), measuring the interferograms from six baselines simultaneously. The instrument has three main components: the IR wavefront sensors; the beam-combiner instrument; and the laser metrology system.
The GRAVITY IR wavefront sensors will be mounted in the Coudé rooms of the UTs and will command the existing Multiple Application Curvature Adaptive Optics (MACAO) deformable mirrors. The system can work on either of the two beams (on-axis or off-axis) behind the PRIMA star separators. Any additional tip/tilt from the beam relay down to the VLTI laboratory will be corrected by a dedicated laser-guiding system. Low frequency drifts of the field and pupil will be corrected by GRAVITY's internal acquisition and guiding camera. The interplay of these systems will guarantee an unperturbed and seeing-corrected beam at the entrance of the beam-combiner instrument in the VLTI laboratory. The interferometric instrument will work on the 2 arsec (for UTs) or 4 arcsec (for ATs) VLTI field of view. Both the reference star and the science object have to lie within this field of view. The light of the two objects from the four telescopes is coupled into optical fibres for modal filtering, to compensate for the differential delay and to adjust the polarisation. The fibres feed two integrated optics beam combiners and the coherently combined light is dispersed in two spectrometers. A low resolution spectrometer provides internal phase- and group-delay tracking on the reference star, and thus enables long exposure times on the science target. Three spectral resolutions with up to R~4000 are implemented in the science spectrometer, and a Wollaston prism provides basic polarimetry.
GRAVITY will measure the visibility of the reference star and the science object simultaneously for all spectral channels, and the differential phase between the two objects. This information will be used for interferometric imaging exploring the complex visibilities, and for astrometry using the differential phase and group delay. All functions of the GRAVITY beam-combiner instrument are implemented in a single cryostat for optimum stability, cleanliness, and thermal background suppression. The internal path lengths of the VLTI and GRAVITY are monitored using dedicated laser metrology. The laser light is back-propagated from the beam combiner and covers the full beam up to the telescope spider above the primary mirror. GRAVITY will make use of high-speed IR photoncountingdetector arrays in both the adaptive optics systems and the fringe tracker. These devices do not suffer from high readoutnoise, which in current IR arrays is tenor more electrons per pixel at framerates of a few hundred Hz.
The GRAVITY system is not a monolithic instrument. 31) It is a collection of subsystems that aims to precisely control the wavefront of the incoming light and its path through the VLTI system before the actual combination of beams takes place and interference fringes are created. A unique aspect of GRAVITY, and the first time this is ever realized, is its ability to interfere the light coming from either a single astronomical source (single-field mode or on-axis) or from two sources simultaneously (dual-field mode or off-axis). In dual-field, the GRAVITY system can perform phase referenced observations supported by the accurate knowledge of the path length which is assessed by a laser system. In this mode of observation, the interferometric phase of the primary star is calibrated to that of the secondary against the detrimental influence of the atmosphere. This enables highly accurate angle measurements on sky and is the basis for GRAVITY's astrometric observing mode. In GRAVITY's imaging observing mode, dual-field observations allow to observe relatively faint targets and use a brighter star as the fringe-track star. The difference between the two modes of GRAVITY is the observation strategy and the use of the laser metrology to measure the light-path.
The subsystems of GRAVITY include:
• The IR wavefront sensing system CIAO (Coudé Infrared Adaptive Optics). CIAO is located in each of the UT coude room and it will operate with the deformable mirror of the MACAOs (Multiple Application Curvature Adaptive Optics).
• A polarization control system to counteract polarization effects in the VLTI. GRAVITY can work either in a split or a combined polarization mode.
• An active pupil guide system including LED sources mounted on each of the telescope spiders.
• A field-guide system to track the position of the source and ensure proper injection into mono-mode fibers.
The overall GRAVITY system interacts and works in harmony with the VLT-I system of telescopes and delay lines. The operation of various GRAVITY subsystems is transparent to the user.
The single and dual field modes. The primary unit of GRAVITY is the Beam Combining Instrument (BCI) that performs the acquisition process and provides the interferometric fringes. The GRAVITY BCI is cryogenically cooled and physically located in the VLT-I laboratory. Within the BCI cryostat, a field is separated and two stars find their way into either the science channel or the fringe-tracking channel. In single-field the FT channel and SC channel receive light from the same star which is split 50%-50%. The GRAVITY fringe-tracker (FT) forms an integral part of the observational approach, i.e. GRAVITY science observations are always done with active fringe-tracking. The FT fringe position is analyzed at a frequency of approximately a kHz in order to correct for the atmospheric and instrumental piston (i.e. a residual optical path difference between beams) by modulating piezo mounted mirrors within the instrument. The FT star thus allows longer detector integration times in the science channel (SC, up to 60 seconds) without compromising the contrast of the fringe pattern. In dual field mode (astrometric or imaging), the fringes of one star are formed in the science channel, and those of the other star in the fringe tracker channel. The two astronomical sources can have an angular separation up to 2 arcsec when observing with the UTs, or 4 arcsec with the ATs. For separations up to 0.4 arcsec and 1.6 arcsec on the UTs and ATs, respectively, the dual-field "on-axis" set-up us used. In this case, the light is split using a beam splitter so that only 50% of the light from each source are injected into the FT and SC channels. In the dual field "off-axis" setup, for larger separations, a roof-top mirror is used and 100% of the light of the FT and SC objects reach the respective fibers.
GRAVITY delivers spectrally dispersed interference fringes that allow stellar interferometry. The FT spectrometer always operates at low spectral resolution (R ~ 22). Taking advantage of the longer integration times, the science channel records the entire K-band at each of the three implemented spectral resolutions of R ~ 22, 500 and 4000. The fringes give access to interferometric quantities such as absolute and differential visibility, spectral differential phase and closure phase. These quantities provide information of physical phenomena at a spatial resolution that can reach 2 mas (depending on the VLT-I baseline), as well as time-resolved differential astrometry at the exquisite accuracy of a few tens of µas.
Figure 21: The Beam Combining Instrument (BCI) with its sub-components labelled (image credit: ESO)
Astrometry and phase-referenced imaging: Astrometry aims at measuring the separation between two targets, whereas phase-referenced imaging aims at measuring the phase of the SC target in reference to the FT target. Both techniques use the dual field mode of GRAVITY and rely on the laser metrology to make the connection between the SC and FT measurements. For astrometry, the metrology measures the evolution of the differential optical path difference as a function of time, which through the interferometer baselines can be converted into a separation between the targets. For phase-referenced imaging the metrology is used to relocate the FT target reference to a separation offset close to the SC target, which helps for producing images. As the laser metrology provides relative optical path measurements only, a metrology zero must be determined for both techniques to work. This determination is typically achieved by swapping a pair of targets (i.e. reversing the sign of the separation), which separates the sidereal metrology signal that changes sign, from the constant metrology zero. When swapping is not directly possible on the target pair of interest, due to a very faint SC target not observable with the FT, the metrology zero determination can be carried out on a more balanced nearby pair. Other than the requirement to calibrate the metrology zero-points, astrometry and phase-referenced imaging observations are similar to dual-field observations and have the same observing constraints.
Whereas the instrument itself is thus perfectly capable of carrying out astrometric and phase-referenced observations, the astrometric part of the GRAVITY pipeline is still under heavy development. The production of separations and referenced phases still rely on custom tools developed by a small group of astrometry-minded astronomers gravitating around the instrument. In addition, the performance of these modes is not fully characterized.
MUSE (Multi Unit Spectroscopic Explorer)
MUSE is the latest of the second-generation instruments to be installed on Yepun (UT4), the fourth Unit Telescope of the Very Large Telescope at the Paranal Observatory. The instrument was built under ESO contract by the MUSE consortium consisting of AIP Potsdam, CRAL Lyon, ESO, EHT Zürich, IRAP Toulouse, Leiden Observatory, and IAG Göttingen. 32)
Like SINFONI, MUSE is an integral field spectrograph (IFS). An IFS allows you to observe the entirety of an astronomical object in one go, and for each pixel measures the intensity of the light as a function of its color, or wavelength. The resulting data is a 3D set where each pixel of the image has a full spectrum of the light. MUSE splits the field of view into 24 individual image segments or channels which are each split further into 48 slices or "mini slits", giving a total of 1152 mini slits. Each set of 48 mini slits is injected into a spectrograph, which disperses the light into its constituent colors, and MUSE measures over 4000 of these colors! From this, the 3D image is created.
"MUSE has been built with the intention of studying the content and processes going on in the very early Universe, when the first stars and galaxies were forming," explains Fernando Selman, Instrument Scientist for MUSE. "Closer in time and space, MUSE will map the dark matter distribution in clusters of galaxies using the gravitational microlensing effect on background galaxies." MUSE will also provide detailed information about the internal dynamics of many classes of galaxies with unprecedented detail. "It has already been used to study the Sombrero Galaxy in Virgo, and, in the same cluster, a recently discovered new type of object — a galaxy being destroyed after falling into the cluster and encountering the cluster's hot gaseous corona," continues Fernando. An image of this galaxy is shown on this page. The stars themselves found within bigger objects will also be a focus of study — impressive vistas of the Tarantula Nebula and its huge collection of massive young stars have been obtained during the testing phase, and an enormous mosaic of the Orion Nebula has been produced.
With nearly 400 million pixels to be processed in realtime, MUSE has presented Paranal Observatory with new computation and communication challenges. During the first phase of commissioning alone, nearly half a billion spectra were produced!
MUSE and adaptive optics: MUSE entered a new era in 2017 with the advanced capabilities of the Adaptive Optics Facility (AOF). The AOF with Artificial laser stars (4LGSF); deformable active mirrors (ann16078); multiple wavefront sensors GALACSI will develop the full potential of MUSE and is comparable to moving the telescope 900 meters above the Paranal summit, a height free of the effects of the most turbulent layers of the atmosphere, giving much clearer images than before.
MUSE was will be the first of the VLT's second generation instruments to taste these new capabilities. The module GALACSI will routinely deliver images to MUSE at optical wavelengths of a quality that was previously possible only on the few clearest nights of the year if at all.
"MUSE with the AOF system will allow for the completion of surveys of the remote Universe with unique sensitivity, permitting studies of the earliest galaxies and large scale structures," says Selman. "During its lifetime we expect many great contributions from this instrument for a wide range of astronomical investigations".
Instrument: MUSE has a modular structure composed of 24 identical IFU modules that together sample, in Wide Field Mode (WFM), a near-contiguous 1 squared arcmin field of view. Spectrally the instrument samples almost the full optical domain with a mean resolution of 3000. Spatially, the instrument samples the sky with 0.2 arcseconds spatial pixels in the currently offered Wide Field Mode with natural seeing (WFM-noAO). 33) 34) 35)
MUSE offers the GLAO (Ground Layer Adaptive Optics) mode of the VLT Adaptive Optics Facility (AOF) via the GALACSI adaptive optics module, offering an AO corrected 1'x1' field of view with 0.2 arcseconds sampling (WFM-AO). Starting P103 offers a 7.5 x 7.5 arcsec2 LTAO corrected field of view sampled at 0.025"/pixel (NFM-AO).
• Wavelength range: 480-930 nm (nominal); 465-930 nm (extended)
• Detectors: 24 x 4 k x 4 k MIT/LL CCD
• AO type: Ground Layer, 4 x (5-10 W) lasers
• Throughput WFM (Wide Field Mode): 14 % (480 nm) 35 % (750 nm) 14 % (930 nm)
• Throughput NFM (Narrow Field Mode): 13 % (480 nm) 26 % (750 nm) 11 % (930 nm)
Figure 22: MUSE) is a second-generation instrument in development for ESO's Very Large Telescope (VLT), due to begin operation in 2012. MUSE is an extremely powerful and innovative 3D spectrograph with a wide field of view, providing simultaneous spectra of numerous adjacent regions in the sky. The instrument is fed by a new multiple-laser adaptive optics system on the VLT. The development of MUSE has been a key experience for the next generation instruments, for both the VLT and the planned Extremely Large Telescope (ELT). The VLT instrumentation program is the most ambitious ever conceived for a single observatory (image credit: ESO)
Sample Observations with VLT (Very Large Telescope) and Status
• March 27, 2019: For the first time, astronomers have succeeded in investigating an exoplanet using optical interferometry. The new method allowed astronomers to measure the position of the exoplanet HR 8799e with unprecedented accuracy. Also, the planet's spectrum was recorded as precisely as never before, paving the way for future searches for life on other planets. The measurements, which were obtained with the participation of astronomers from the Max Planck Institutes for Astronomy and for Extraterrestrial Physics, were performed with the GRAVITY instrument at ESO's Paranal Observatory. 36)
- Investigating exoplanets in detail and without confounding noise is difficult. In general, with increasing distance, it becomes more and more difficult to image fine details of an astronomical object. Furthermore, exoplanets are typically buried in the glare of their much brighter host stars. Now, a group of researchers led by Sylvestre Lacour of the Observatoire de Paris and the Max Planck Institute for Extraterrestrial Physics, also including MPIA researchers, has been able to demonstrate a new method of investigation that mitigates these problems and thereby provides a new perspective on exoplanets.
- Key to the new technique is the GRAVITY instrument, which has been in operation at the European Southern Observatory's Very Large Telescope Interferometer (VLTI) at Paranal Observatory in Chile since 2016. Using a technique known as interferometry, which exploits the wave nature of light, GRAVITY is able to combine the light of several telescopes to form a common image. Combined, the four 8-metre-telescopes of the Very Large Telescope (VLT) can make images so detailed that a single telescope would need to have a mirror diameter of approximately 100 meters to provide the same level of detail.
- The study of the exoplanet HR 8799e that has now been published is the first to demonstrate the potential of interferometric observations for the investigation of exoplanets in practice. The planet is one of only a few (about 120 out of 4000) for which direct images exist; so far, most exoplanets have only been detected indirectly. HR 8977e is part of a young five-body-system, a mere 130 light-years away from us, which consists of the star HR 8799 and four planets (as far as we know, at least). All of the planets are gas giants with between 5 and 10 times the mass of Jupiter.
- This result was announced today in a letter in the journal Astronomy and Astrophysics by the GRAVITY Collaboration, in which they present observations of the exoplanet HR8799e using optical interferometry. The exoplanet was discovered in 2010 orbiting the young main-sequence star HR8799, which lies around 129 light-years from Earth in the constellation of Pegasus. 37) 38)
- Today's result, which reveals new characteristics of HR8799e, required an instrument with very high resolution and sensitivity. GRAVITY can use ESO's VLT's four unit telescopes to work together to mimic a single larger telescope using a technique known as interferometry. This creates a super-telescope — the VLTI — that collects and precisely disentangles the light from HR8799e's atmosphere and the light from its parent star.
- HR8799e is a ‘super-Jupiter', a world unlike any found in our Solar System, that is both more massive and much younger than any planet orbiting the Sun. At only 30 million years old, this baby exoplanet is young enough to give scientists a window onto the formation of planets and planetary systems. The exoplanet is thoroughly inhospitable — leftover energy from its formation and a powerful greenhouse effect heat HR8799e to a hostile temperature of roughly 1000 °C.
- This is the first time that optical interferometry has been used to reveal details of an exoplanet, and the new technique furnished an exquisitely detailed spectrum of unprecedented quality — ten times more detailed than earlier observations. The team's measurements were able to reveal the composition of HR8799e's atmosphere — which contained some surprises.
Figure 23: This artist's impression shows the observed exoplanet, which goes by the name HR8799e. The GRAVITY instrument on ESO's Very Large Telescope Interferometer (VLTI) has made the first direct observation of an exoplanet using optical interferometry. This method revealed a complex exoplanetary atmosphere with clouds of iron and silicates swirling in a planet-wide storm. The technique presents unique possibilities for characterizing many of the exoplanets known today. (image credit: ESO, Luis Calçada)
- "Our analysis showed that HR8799e has an atmosphere containing far more carbon monoxide than methane — something not expected from equilibrium chemistry," explains team leader Sylvestre Lacour researcher CNRS at the Observatoire de Paris - PSL and the Max Planck Institute for Extraterrestrial Physics. "We can best explain this surprising result with high vertical winds within the atmosphere preventing the carbon monoxide from reacting with hydrogen to form methane."
- The team found that the atmosphere also contains clouds of iron and silicate dust. When combined with the excess of carbon monoxide, this suggests that HR8799e's atmosphere is engaged in an enormous and violent storm.
- "Our observations suggest a ball of gas illuminated from the interior, with rays of warm light swirling through stormy patches of dark clouds," elaborates Lacour. "Convection moves around the clouds of silicate and iron particles, which disaggregate and rain down into the interior. This paints a picture of a dynamic atmosphere of a giant exoplanet at birth, undergoing complex physical and chemical processes."
- This result builds on GRAVITY's string of impressive discoveries, which have included breakthroughs such as last year's observation of gas swirling at 30% of the speed of light just outside the event horizon of the massive Black Hole in the Galactic Centre. It also adds a new way of observing exoplanets to the already extensive arsenal of methods available to ESO's telescopes and instruments — paving the way to many more impressive discoveries.
• March 14, 2019: ESO's Very Large Telescope (VLT) has caught a glimpse of an ethereal nebula hidden away in the darkest corners of the constellation of Orion (The Hunter) — NGC 1788, nicknamed the Cosmic Bat. This bat-shaped reflection nebula doesn't emit light — instead it is illuminated by a cluster of young stars in its core, only dimly visible through the clouds of dust. Scientific instruments have come a long way since NGC 1788 was first described, and this image taken by the VLT is the most detailed portrait of this nebula ever taken. 39)
- Even though this ghostly nebula in Orion appears to be isolated from other cosmic objects, astronomers believe that it was shaped by powerful stellar winds from the massive stars beyond it. These streams of scorching plasma are thrown from a star's upper atmosphere at incredible speeds, shaping the clouds secluding the Cosmic Bat's nascent stars.
- NGC 1788 was first described by the German–British astronomer William Herschel, who included it in a catalogue that later served as the basis for one of the most significant collections of deep-sky objects, the New General Catalogue (NGC). A nice image of this small and dim nebula had already been captured by the MPG/ESO 2.2-meter telescope at ESO's La Silla Observatory, but this newly observed scene leaves it in the proverbial dust. Frozen in flight, the minute details of this Cosmic Bat's dusty wings were imaged for the twentieth anniversary of one of ESO's most versatile instruments, the FOcal Reducer and low dispersion Spectrograph 2 (FORS2).
- FORS2 is an instrument mounted on Antu, one of the VLT's 8.2-meter Unit Telescopes at the Paranal Observatory, and its ability to image large areas of the sky in exceptional detail has made it a coveted member of ESO's fleet of cutting-edge scientific instruments. Since its first light 20 years ago, FORS2 has become known as "the Swiss army knife of instruments". This moniker originates from its uniquely broad set of functions . FORS2's versatility extends beyond purely scientific uses — its ability to capture beautiful high-quality images like this makes it a particularly useful tool for public outreach.
- This image was taken as part of ESO's Cosmic Gems program, an outreach initiative that uses ESO telescopes to produce images of interesting, intriguing or visually attractive objects for the purposes of education and public outreach. The program makes use of telescope time that cannot be used for science observations, and — with the help of FORS2 — produces breathtaking images of some of the most striking objects in the night sky, such as this intricate reflection nebula. In case the data collected could be useful for future scientific purposes, these observations are saved and made available to astronomers through the ESO Science Archive.
Figure 24: Hidden in one of the darkest corners of the Orion constellation, this Cosmic Bat is spreading its hazy wings through interstellar space two thousand light-years away. It is illuminated by the young stars nestled in its core — despite being shrouded by opaque clouds of dust, their bright rays still illuminate the nebula. Too dim to be discerned by the naked eye, NGC 1788 reveals its soft colors to ESO's Very Large Telescope in this image — the most detailed to date (image credit: ESO)
Figure 25: Hidden in one of the darkest corners of the Orion constellation, this Cosmic Bat is spreading its hazy wings through interstellar space two thousand light-years away. It is illuminated by the young stars nestled in its core — despite being shrouded by opaque clouds of dust, their bright rays still illuminate the nebula. Too dim to be discerned by the naked eye, NGC 1788 reveals its soft colors to ESO's Very Large Telescope in this image — the most detailed to date (video credit: ESO)
• February 6, 2019: This region of the Large Magellanic Cloud (LMC) glows in striking colors in this image (Figure 26) captured by the Multi Unit Spectroscopic Explorer (MUSE) instrument on ESO's Very Large Telescope (VLT). The region, known as LHA 120-N 180B - N180 B for short - is a type of nebula known as an H II region (pronounced "H two"), and is a fertile source of new stars. 40) 41)
- The LMC is a satellite galaxy of the Milky Way, visible mainly from the Southern Hemisphere. At only around 160,000 light-years away from the Earth, it is practically on our doorstep. As well as being close to home, the LMC's single spiral arm appears nearly face-on, allowing us to inspect regions such as N180 B with ease.
- H II regions are interstellar clouds of ionized hydrogen — the bare nuclei of hydrogen atoms. These regions are stellar nurseries — and the newly formed massive stars are responsible for the ionization of the surrounding gas, which makes for a spectacular sight. N180 B's distinctive shape is made up of a gargantuan bubble of ionized hydrogen surrounded by four smaller bubbles.
- Deep within this glowing cloud, MUSE has spotted a jet emitted by a fledgling star — a massive young stellar object with a mass 12 times greater than our Sun. The jet — named Herbig–Haro 1177, or HH 1177 for short — is shown in detail in this accompanying image. This is the first time such a jet has been observed in visible light outside the Milky Way, as they are usually obscured by their dusty surroundings. However, the relatively dust-free environment of the LMC allows HH 1177 to be observed at visible wavelengths. At nearly 33 light-years in length, it is one of the longest such jets ever observed.
- HH 1177 tells us about the early lives of stars. The beam is highly collimated; it barely spreads out as it travels. Jets like this are associated with the accretion discs of their star, and can shed light on how fledgling stars gather matter. Astronomers have found that both high- and low-mass stars launch collimated jets like HH 1177 via similar mechanisms — hinting that massive stars can form in the same way as their low-mass counterparts.
- MUSE has recently been vastly improved by the addition of the Adaptive Optics Facility, the Wide Field Mode of which saw first light in 2017. Adaptive optics is the process by which ESO's telescopes compensate for the blurring effects of the atmosphere — turning twinkling stars into sharp, high-resolution images. Since obtaining these data, the addition of the Narrow Field Mode, has given MUSE vision nearly as sharp as that of the NASA/ESA Hubble Space Telescope — giving it the potential to explore the Universe in greater detail than ever before.
Figure 26: This dazzling region of newly-forming stars in the Large Magellanic Cloud (LMC) was captured by the MUSE (Multi Unit Spectroscopic Explorer) instrument on ESO's Very Large Telescope. The relatively small amount of dust in the LMC and MUSE's acute vision allowed intricate details of the region to be picked out in visible light (image credit: ESO, A McLeod, et al.)
• November 29, 2018: Detailed observations of the quasar 3C 273 with the GRAVITY instrument reveal the structure of rapidly moving gas around the central super-massive black hole, the first time that the so-called "broad line region" could be resolved. The international team of astronomers was thus able to measure the mass of the black hole with unprecedented precision. This measurement confirms the fundamental assumptions of the most commonly used method to measure the mass of central black holes in distant quasars. Studying these black holes and determining their masses is an essential ingredient to understanding galaxy evolution in general. 42)
- An international team of astronomers has now used the GRAVITY instrument to look deep into the heart of the quasar and was able to actually observe the structure of rapidly moving gas around the central black hole. So far, such observations had not been possible due to the small angular size of this inner region, which is about the size of our Solar system but at a distance of some 2.5 billion light years. The GRAVITY instrument combines all four ESO VLT telescopes in a technique called interferometry, which allows a huge gain in angular resolution, equivalent to a telescope with 130 meters in diameter. Thus the astronomers can reveal structures at the level of 10 µas (micro-arcsec), which corresponds to about 0.1 light years at the distance of the quasar (or an object the size of a 1-Euro-coin on the Moon).
- "GRAVITY allowed us to resolve the so-called ‘broad line region' for the first time ever, and to observe the motion of gas clouds around the central black hole", explains Eckhard Sturm, lead author from the Max Planck Institute for Extraterrestrial Physics (MPE). "Our observations reveal that the gas clouds do whirl around the central black hole." 43)
- The broad atomic emission lines are an observational hallmark of quasars, clearly indicating the extra-galactic origin of the source. So far, the size of the broad line region is measured mainly by a method called "reverberation mapping". Brightness variations of the quasar's central engine cause a light echo once the radiation hits clouds further out – the larger the size of the system, the later the echo. In the best cases, the motions of the gas can also be identified, often implying a disk in rotation. This result, derived from timing information, can now be confronted with spatially resolved observations with GRAVITY.
- "Our results support the fundamental assumptions of reverberation mapping," confirms Jason Dexter, co-lead author from MPE. "Information about the motion and size of the region immediately around the black hole are crucial to measure its mass," he adds. For the first time, the method was now tested experimentally and passed its test with flying colors, confirming previous mass estimates of about 300 million solar masses for the black hole. Thus, GRAVITY provides both a confirmation of the main method used previously to determine black hole masses in quasars and a new and highly accurate, independent method to measure such masses. It thereby promises to provide a benchmark for measuring black hole masses in thousands of other quasars.
- Quasars play a fundamental role in the history of the Universe, as their evolution is intricately tied to galaxy growth. While astronomers assume that basically all large galaxies harbor a massive black hole at their center, so far only the one in our Milky Way has been accessible for detailed studies.
- "This is the first time that we can spatially resolve and study the immediate environs of a massive black hole outside our home galaxy, the Milky Way," emphasizes Reinhard Genzel, head of the infrared research group at MPE. "Black holes are intriguing objects, allowing us to probe physics under extreme conditions – and with GRAVITY we can now probe them both near and far."
Figure 27: This animation shows the zoom from an optical image of the quasar to an artistic representation of the surroundings of a supermassive black hole. There is a dusty ring of very hot material collapsing onto the gravity trap and often a jet in which material is ejected at high velocities at the poles. Astronomers have now succeeded in spatially depicting the so-called broad line region, in which gas clouds swirl around the central black hole (video credit: L. Calcada/ESO)
Figure 28: Powerhouse in space: The quasar 3C273 resides in a giant elliptical galaxy in the constellation of Virgo at a distance of about 2.5 billion light years. It was the first quasar ever to be identified (image credit: ESA/Hubble & NASA)
• November 19, 2018: University of Sydney astronomers, working with international colleagues, have found a star system like none seen before in our galaxy. The scientists believe one of the stars—about 8000 light years from Earth—is the first known candidate in the Milky Way to produce a dangerous gamma-ray burst, among the most energetic events in the universe, when it explodes and dies. 44)
Figure 29: This is an image of Apep captured at 8 µm in the thermal infrared with the VISIR camera on the European Southern Observatory's VLT telescope, Mt Paranal, Chile. The system can be seen to be a binary, with a much fainter companion to the North of the heart of the system. This companion is not believed to play a role in the sculping of the extended dust plume, about 12 arcseconds across. The origin of this structure comes from the central region, believed itself to contain a binary (the whole thing being a triple star), image credit: Peter Tuthill/University of Sydney/ESO
- The system, comprising a pair of scorchingly luminous stars, was nicknamed Apep by the team after the serpentine Egyptian god of chaos. One star is on the brink of a massive supernova explosion.
- The findings, published today in Nature Astronomy, are controversial as no gamma-ray burst has ever been detected within our own galaxy, the Milky Way. 45)
- Yet in the southern constellation of Norma, nestled just beneath Scorpio's tail, astronomers have discovered this uniquely beautiful star system.
- At its heart, wrapped in an elegantly sculpted plume of dust and gas, lies a powerful binary pair.
- The two hot, luminous stars—known to astronomers as Wolf-Rayets - orbit each other every hundred years or so, according to the research conducted at the Sydney Institute for Astronomy.
- This orbital dance is embossed on a fast wind streaming off the stars. Using spectroscopy, the astronomers have measured the velocity of the stellar winds as fast as 12 million km/hr, about 1 percent the speed of light.
Figure 30: This animated gif is intended to illustrate the geometry of the structure that we have witnessed in the Apep system. From a single image, it is harder to understand the 3-D structure. The central binary (only: not the wider Northern companion in the triple) is illustrated as the blue star at the center. The geometry given is that believed typical for a Wolf-Rayet colliding pinwheel system: that is an optically thin dust plume distributed over the surface of a cone that is dictated by the colliding winds. The whole outflow structure is wrapped into a spiral by the orbital motion of the presumed central binary. Further the dust formation has a specific onset and cessation, which truncate the spiral at the outer and inner limits (for example, giving rise to the notable elliptical hole). Note this is a toy animation to illustrate a fly-around of the structure, and not a model fitted to the data that describes the dust flow process. The looping animation proceeds for about half an orbit (say roughly 60 years) with a pause at about the present epoch. Note that the motion we actually recorded with VISIR in the real data only spans 3 years (image credit: Peter Tuthill/University of Sydney/ESO)
- Dr. Joe Callingham, lead author of the study, said: "We discovered this star as an outlier in a survey with a radio telescope operated by the University of Sydney. We knew immediately we had found something quite exceptional: the luminosity across the spectrum from the radio to the infrared was off the charts." Dr. Callingham is now at the Netherlands Institute for Radio Astronomy. — "When we saw the stunning dust plume coiled around the these incandescent stars, we decided to name it 'Apep' - the monstrous serpent deity and mortal enemy of Sun god Ra from Egyptian mythology."
- That sculpted plume is what makes the system so important, said Professor Peter Tuthill, research group leader at the University of Sydney. "When we saw the spiral dust tail we immediately knew we were dealing with a rare and special kind of nebula called a pinwheel," Professor Tuthill said. "The curved tail is formed by the orbiting binary stars at the center, which inject dust into the expanding wind creating a pattern like a rotating lawn sprinkler. Because the wind expands so much, it inflates the tiny coils of dust revealing the physics of the stars at the heart of the system."
- However, the data on the plume presented a conundrum: the stellar winds were expanding 10 times faster than the dust.
- "It was just astonishing," Professor Tuthill said. "It was like finding a feather caught in a hurricane just drifting along at walking pace."
- Dr. Benjamin Pope, a co-author from New York University, said: "The key to understanding the bizarre behavior of the wind lies in the rotation of the central stars. - What we have found in the Apep system is a supernova precursor that seems to be very rapidly rotating, so fast it might be near break-up."
- Wolf-Rayet stars, like those driving Apep's plume, are known to be very massive stars at the ends of their lives; they could explode as supernovae at any time.
- "The rapid rotation puts Apep into a whole new class. Normal supernovae are already extreme events but adding rotation to the mix can really throw gasoline on the fire."
- The researchers think this might be the recipe for a perfect stellar storm to produce a gamma-ray burst, which are the most extreme events in the Universe after the Big Bang itself. Fortunately, Apep appears not to be aimed at Earth, because a strike by a gamma-ray burst from this proximity could strip ozone from the atmosphere, drastically increasing our exposure to UV light from the Sun.
- "Ultimately, we can't be certain what the future has in store for Apep," Professor Tuthill said. "The system might slow down enough so it explodes as a normal supernova rather than a gamma-ray burst. However, in the meantime, it is providing astronomers a ringside seat into beautiful and dangerous physics that we have not seen before in our galaxy."
• October 31, 2018: ESO's exquisitely sensitive GRAVITY instrument has added further evidence to the long-standing assumption that a supermassive black hole lurks in the center of the Milky Way. New observations show clumps of gas swirling around at about 30% of the speed of light on a circular orbit just outside its event horizon — the first time material has been observed orbiting close to the point of no return, and the most detailed observations yet of material orbiting this close to a black hole. 46)
Figure 31: ESO's GRAVITY instrument on the VLT Interferometer has been used by scientists from a consortium of European institutions, including ESO, to observe flares of infrared radiation coming from the accretion disc around Sagittarius A*, the massive object at the heart of the Milky Way. The observed flares provide long-awaited confirmation that the object in the center of our galaxy is, as has long been assumed, a supermassive black hole. The flares originate from material orbiting very close to the black hole's event horizon — making these the most detailed observations yet of material orbiting this close to a black hole (image credit: MPE Garching, Observatoire de Paris, Université Grenoble Alpes, CNRS, Max Planck Institute for Astronomy, University of Cologne, Portuguese CENTRA – Centro de Astroﬁsica e Gravitação and ESO)
- While some matter in the accretion disc — the belt of gas orbiting Sagittarius A* at relativistic speeds — can orbit the black hole safely, anything that gets too close is doomed to be pulled beyond the event horizon. The closest point to a black hole that material can orbit without being irresistibly drawn inwards by the immense mass is known as the innermost stable orbit, and it is from here that the observed flares originate.
- "It's mind-boggling to actually witness material orbiting a massive black hole at 30% of the speed of light," marvelled Oliver Pfuhl, a scientist at the MPE. "GRAVITY's tremendous sensitivity has allowed us to observe the accretion processes in realtime in unprecedented detail."
- These measurements were only possible thanks to international collaboration and state-of-the-art instrumentation. The GRAVITY instrument which made this work possible combines the light from four telescopes of ESO's VLT to create a virtual super-telescope 130 meters in diameter, and has already been used to probe the nature of Sagittarius A*.
- Earlier this year, GRAVITY and SINFONI, another instrument on the VLT, allowed the same team to accurately measure the close fly-by of the star S2 as it passed through the extreme gravitational field near Sagittarius A*, and for the first time revealed the effects predicted by Einstein's general relativity in such an extreme environment. During S2's close fly-by, strong infrared emission was also observed.
- "We were closely monitoring S2, and of course we always keep an eye on Sagittarius A*," explained Pfuhl. "During our observations, we were lucky enough to notice three bright flares from around the black hole — it was a lucky coincidence!"
- This emission, from highly energetic electrons very close to the black hole, was visible as three prominent bright flares, and exactly matches theoretical predictions for hot spots orbiting close to a black hole of four million solar masses. The flares are thought to originate from magnetic interactions in the very hot gas orbiting very close to Sagittarius A*.
- Reinhard Genzel, of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, who led the study, explained: "This always was one of our dream projects but we did not dare to hope that it would become possible so soon." Referring to the long-standing assumption that Sagittarius A* is a supermassive black hole, Genzel concluded that "the result is a resounding confirmation of the massive black hole paradigm." 47)
• October 01, 2018: An unexpected abundance of Lyman-alpha emission in the HUDF (Hubble Ultra Deep Field) region was discovered by an international team of astronomers using the MUSE (Multi Unit Spectroscopic Explorer) instrument on ESO's VLT (Very Large Telescope). The discovered emission covers nearly the entire field of view — leading the team to extrapolate that almost all of the sky is invisibly glowing with Lyman-alpha emission from the early Universe. 48) 49)
- Astronomers have long been accustomed to the sky looking wildly different at different wavelengths, but the extent of the observed Lyman-alpha emission was still surprising. "Realizing that the whole sky glows in optical when observing the Lyman-alpha emission from distant clouds of hydrogen was a literally eye-opening surprise," explained Kasper Borello Schmidt, a member of the team of astronomers behind this result.
- "This is a great discovery!" added team member Themiya Nanayakkara. "Next time you look at the moonless night sky and see the stars, imagine the unseen glow of hydrogen: the first building block of the universe, illuminating the whole night sky."
- The HUDF region the team observed is an otherwise unremarkable area in the constellation of Fornax (the Furnace), which was famously mapped by the NASA/ESA Hubble Space Telescope in 2004, when Hubble spent more than 270 hours of precious observing time looking deeper than ever before into this region of space.
- The HUDF observations revealed thousands of galaxies scattered across what appeared to be a dark patch of sky, giving us a humbling view of the scale of the Universe. Now, the outstanding capabilities of MUSE have allowed us to peer even deeper. The detection of Lyman-alpha emission in the HUDF is the first time astronomers have been able to see this faint emission from the gaseous envelopes of the earliest galaxies. This composite image shows the Lyman-alpha radiation in blue superimposed on the iconic HUDF image.
- MUSE, the instrument behind these latest observations, is a state-of-the-art integral field spectrograph installed on Unit Telescope 4 of the VLT at ESO's Paranal Observatory.
When MUSE observes the sky, it sees the distribution of wavelengths in the light striking every pixel in its detector. Looking at the full spectrum of light from astronomical objects provides us with deep insights into the astrophysical processes occurring in the Universe.
Figure 32: Deep observations made with the MUSE spectrograph on ESO's VLT have uncovered vast cosmic reservoirs of atomic hydrogen surrounding distant galaxies. The exquisite sensitivity of MUSE allowed for direct observations of dim clouds of hydrogen glowing with Lyman-alpha emission in the early Universe — revealing that almost the whole night sky is invisibly aglow (image credit: eso 1832)
• September 12, 2018: This wonderful image shows the resplendent spiral galaxy NGC 3981 suspended in the inky blackness of space. This galaxy, which lies in the constellation of Crater (the Cup), was imaged in May 2018 using the FOcal Reducer and low dispersion Spectrograph 2 (FORS2) instrument on ESO's Very Large Telescope (VLT). 50)
- FORS2 is mounted on Unit Telescope 1 (Antu) of the VLT at ESO's Paranal Observatory in Chile. Amongst the host of cutting-edge instruments mounted on the four Unit Telescopes of the VLT, FORS2 stands apart due to its extreme versatility. This "Swiss Army knife" of an instrument is able to study a variety of astronomical objects in many different ways — as well as being capable of producing beautiful images like this one.
- The sensitive gaze of FORS2 revealed NGC 3981's spiral arms, strewn with vast streams of dust and star-forming regions, and a prominent disc of hot young stars. The galaxy is inclined towards Earth, allowing astronomers to peer right into the heart of this galaxy and observe its bright center, a highly energetic region containing a supermassive black hole. Also shown is NGC 3981's outlying spiral structure, some of which appears to have been stretched outwards from the galaxy, presumably due to the gravitational influence of a past galactic encounter.
- NGC 3981 certainly has many galactic neighbors. Lying approximately 65 million light years from Earth, the galaxy is part of the NGC 4038 group, which also contains the well-known interacting Antennae Galaxies. This group is part of the larger Crater Cloud, which is itself a smaller component of the Virgo Supercluster, the titanic collection of galaxies that hosts our own Milky Way galaxy.
- NGC 3981 is not the only interesting feature captured in this image. As well as several foreground stars from our own galaxy, the Milky Way, FORS2 also captured a rogue asteroid streaking across the sky, visible as the faint line towards the top of the image. This particular asteroid has unwittingly illustrated the process used to create astronomical images, with the three different exposures making up this image displayed in the blue, green and red sections of the asteroid's path.
- This image was taken as part of ESO's Cosmic Gems program, an outreach initiative to produce images of interesting, intriguing or visually attractive objects using ESO telescopes, for the purposes of education and public outreach. The program makes use of telescope time that cannot be used for science observations. In case the data collected could be useful for future scientific purposes, these observations are saved and made available to astronomers through ESO's science archive.
Figure 33: FORS2, an instrument mounted on ESO's Very Large Telescope, has observed the spiral galaxy NGC 3981 in all its glory. The image was captured as part of the ESO Cosmic Gems Program, which makes use of the rare occasions when observing conditions are not suitable for gathering scientific data. Instead of sitting idle, the ESO Cosmic Gems Program allows ESO's telescopes to be used to capture visually stunning images of the southern skies (image credit: ESO)
• August 8, 2018: Whereas ESO's VLT (Very Large Telescope) can observe very faint astronomical objects in great detail, when astronomers want to understand how the huge variety of galaxies come into being they must turn to a different sort of telescope with a much bigger field of view. The VST (VLT Survey Telescope) is such a telescope. It was designed to explore vast swathes of the pristine Chilean night skies, offering astronomers detailed astronomical surveys of the southern hemisphere. 51) 52)
- The powerful surveying properties of the VST led an international team of astronomers to conduct the VST Early-type GAlaxy Survey (VEGAS)  to examine a collection of elliptical galaxies in the southern hemisphere . Using the sensitive OmegaCAM detector at the heart of the VST , a team led by Marilena Spavone from INAF-Astronomical Observatory of Capodimonte in Naples, Italy, captured images of a wide variety of such galaxies in different environments.
- One of these galaxies is NGC 5018, the milky-white galaxy near the center of this image. It lies in the constellation of Virgo (The Virgin) and may at first resemble nothing but a diffuse blob. But, on closer inspection, a tenuous stream of stars and gas — a tidal tail — can be seen stretching outwards from this elliptical galaxy. Delicate galactic features such as tidal tails and stellar streams are hallmarks of galactic interactions, and provide vital clues to the structure and dynamics of galaxies.
- As well as the many elliptical (and a few spiral) galaxies in this remarkable 400-megapixel image, a colorful variety of bright foreground stars in our own Milky Way Galaxy also pepper the image. These stellar interlopers, such as the vividly blue HD 114746 near the center of the image, are not the intended subjects of this astronomical portrait, but happen to lie between the Earth and the distant galaxies under study. Less prominent, but no less fascinating, are the faint tracks left by asteroids in our own Solar System. Just below NGC 5018, the faint streak left by the asteroid 2001 TJ21 (110423) — captured over several successive observations — can be seen stretching across the image. Further to the right, another asteroid — 2000 WU69 (98603) — left its trace in this spectacular image.
- While astronomers set out to investigate the delicate features of distant galaxies millions of light-years from Earth, in the process they also captured images of nearby stars hundreds of light-years away, and even the faint trails of asteroids only light-minutes away in our own Solar System. Even when studying the furthest reaches of the cosmos, the sensitivity of ESO telescopes and dark Chilean skies can offer entrancing observations much closer to home.
Figure 34: A glittering host of galaxies populate this rich image taken with ESO's VST (VLT Survey Telescope), a state-of-the-art 2.6-m telescope designed for surveying the sky in visible light. The features of the multitude of galaxies strewn across the image allow astronomers to uncover the most delicate details of galactic structure (image credit: eso1827 — Photo Release)
• July 26, 2018: First Successful Test of Einstein's General Relativity Near Supermassive Black Hole. Obscured by thick clouds of absorbing dust, the closest supermassive black hole to the Earth lies 26 000 light-years away at the center of the Milky Way. This gravitational monster, which has a mass four million times that of the Sun, is surrounded by a small group of stars orbiting around it at high speed. This extreme environment — the strongest gravitational field in our galaxy — makes it the perfect place to explore gravitational physics, and particularly to test Einstein's general theory of relativity. 53)
Figure 35: Artist's impression of S2 passing supermassive black hole at the center of the Milky Way. Observations made with ESO's VLT (Very Large Telescope) have for the first time revealed the effects predicted by Einstein's general relativity on the motion of a star passing through the extreme gravitational field near the supermassive black hole in the center of the Milky Way. This long-sought result represents the climax of a 26-year-long observation campaign using ESO's telescopes in Chile (image credit: ESO, M. Kommesser)
- New infrared observations from the exquisitely sensitive GRAVITY, SINFONI and NACO instruments on ESO's VLT have now allowed astronomers to follow one of these stars, called S2, as it passed very close to the black hole during May 2018. At the closest point this star was at a distance of less than 20 billion kilometers from the black hole and moving at a speed in excess of 25 million kilometers per hour — almost three percent of the speed of light. 54)
Figure 36: Orbit diagram of S2 around the supermassive black hole at the center of the Milky Way. It was compiled from observations with ESO telescopes and instruments over a period of more than 25 years. The star takes 16 years to complete one orbit and was very close to the black hole in May 2018 (image credit: ESO/MPE/GRAVITY Collaboration)
- The team compared the position and velocity measurements from GRAVITY and SINFONI respectively, along with previous observations of S2 using other instruments, with the predictions of Newtonian gravity, general relativity and other theories of gravity. The new results are inconsistent with Newtonian predictions and in excellent agreement with the predictions of general relativity.
- These extremely precise measurements were made by an international team led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, in conjunction with collaborators around the world, at the Paris Observatory–PSL, the Université Grenoble Alpes, CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the Portuguese CENTRA – Centro de Astroﬁsica e Gravitação and ESO. The observations are the culmination of a 26-year series of ever-more-precise observations of the center of the Milky Way using ESO instruments.
Figure 37: Cosmic swarm of bees: This simulation shows the orbits of stars very close to the supermassive black hole at the heart of the Milky Way. One of these stars, named S2, orbits every 16 years and is passing very close to the black hole in May 2018 (image credit: ESO/L. Calçada/spaceengine.org)
- "This is the second time that we have observed the close passage of S2 around the black hole in our galactic center. But this time, because of much improved instrumentation, we were able to observe the star with unprecedented resolution," explains Genzel. "We have been preparing intensely for this event over several years, as we wanted to make the most of this unique opportunity to observe general relativistic effects."
- The new measurements clearly reveal an effect called gravitational redshift. Light from the star is stretched to longer wavelengths by the very strong gravitational field of the black hole. And the change in the wavelength of light from S2 agrees precisely with that predicted by Einstein's theory of general relativity. This is the first time that this deviation from the predictions of the simpler Newtonian theory of gravity has been observed in the motion of a star around a supermassive black hole.
- The team used SINFONI to measure the velocity of S2 towards and away from Earth and the GRAVITY instrument in the VLT Interferometer (VLTI) to make extraordinarily precise measurements of the changing position of S2 in order to define the shape of its orbit. GRAVITY creates such sharp images that it can reveal the motion of the star from night to night as it passes close to the black hole — 26 000 light-years from Earth.
- "Our first observations of S2 with GRAVITY, about two years ago, already showed that we would have the ideal black hole laboratory," adds Frank Eisenhauer (MPE), Principal Investigator of GRAVITY and the SINFONI spectrograph. "During the close passage, we could even detect the faint glow around the black hole on most of the images, which allowed us to precisely follow the star on its orbit, ultimately leading to the detection of the gravitational redshift in the spectrum of S2."
- More than one hundred years after he published his paper setting out the equations of general relativity, Einstein has been proved right once more — in a much more extreme laboratory than he could have possibly imagined!
- Françoise Delplancke, head of the System Engineering Department at ESO, explains the significance of the observations: "Here in the Solar System we can only test the laws of physics now and under certain circumstances. So it's very important in astronomy to also check that those laws are still valid where the gravitational fields are very much stronger."
- Continuing observations are expected to reveal another relativistic effect very soon — a small rotation of the star's orbit, known as Schwarzschild precession — as S2 moves away from the black hole.
- Xavier Barcons, ESO's Director General, concludes: "ESO has worked with Reinhard Genzel and his team and collaborators in the ESO Member States for over a quarter of a century. It was a huge challenge to develop the uniquely powerful instruments needed to make these very delicate measurements and to deploy them at the VLT in Paranal. The discovery announced today is the very exciting result of a remarkable partnership."
Figure 38: This diagram shows the motion of the star S2 as it passes close to the supermassive black hole at the center of the Milky Way. It was compiled from observations with the GRAVITY instrument in the VLT interferometer. At this point the star was travelling at nearly 3% of the speed of light and its shift in position can be seen from night to night. The sizes of the star and the black hole are not to scale (image credit: ESO/MPE/GRAVITY Collaboration) 55)
• July 18,2018: ESO's VLT has achieved first light with a new adaptive optics mode called laser tomography — and has captured remarkably sharp test images of the planet Neptune, star clusters and other objects. The pioneering MUSE instrument in Narrow-Field Mode, working with the GALACSI adaptive optics module, can now use this new technique to correct for turbulence at different altitudes in the atmosphere. It is now possible to capture images from the ground at visible wavelengths that are sharper than those from the NASA/ESA Hubble Space Telescope. The combination of exquisite image sharpness and the spectroscopic capabilities of MUSE will enable astronomers to study the properties of astronomical objects in much greater detail than was possible before. 56)
Figure 39: This image of the planet Neptune was obtained during the testing of the Narrow-Field adaptive optics mode of the MUSE/GALACSI instrument on ESO's Very Large Telescope. The corrected image is sharper than a comparable image from the NASA/ESA Hubble Space Telescope (image credit: ESO/P. Weilbacher (AIP))
Figure 40: Neptune from the VLT with and without adaptive optics (image credit: ESO)
- The MUSE (Multi Unit Spectroscopic Explorer) instrument on ESO's Very Large Telescope (VLT) works with an adaptive optics unit called GALACSI (Ground Atmospheric Layer Adaptive Optics for Spectroscopic Imaging). This makes use of the Laser Guide Star Facility, 4LGSF, a subsystem of the AOF (Adaptive Optics Facility). The AOF provides adaptive optics for instruments on the VLT's Unit Telescope 4 (UT4). MUSE was the first instrument to benefit from this new facility and it now has two adaptive optics modes — the Wide Field Mode and the Narrow Field Mode.
- The MUSE Wide Field Mode coupled to GALACSI in ground-layer mode corrects for the effects of atmospheric turbulence up to 1 km above the telescope over a comparatively wide field of view. But the new Narrow Field Mode using laser tomography corrects for almost all of the atmospheric turbulence above the telescope to create much sharper images, but over a smaller region of the sky.
- With this new capability, the 8 m UT4 reaches the theoretical limit of image sharpness and is no longer limited by atmospheric blur. This is extremely difficult to attain in the visible and gives images comparable in sharpness to those from the NASA/ESA Hubble Space Telescope. It will enable astronomers to study in unprecedented detail fascinating objects such as supermassive black holes at the centers of distant galaxies, jets from young stars, globular clusters, supernovae, planets and their satellites in the Solar System and much more.
- Adaptive optics is a technique to compensate for the blurring effect of the Earth's atmosphere, also known as astronomical seeing, which is a big problem faced by all ground-based telescopes. The same turbulence in the atmosphere that causes stars to twinkle to the naked eye results in blurred images of the Universe for large telescopes. Light from stars and galaxies becomes distorted as it passes through our atmosphere, and astronomers must use clever technology to improve image quality artificially.
- To achieve this, four brilliant lasers are fixed to UT4 that project columns of intense orange light 30 cm in diameter into the sky, stimulating sodium atoms high in the atmosphere and creating artificial Laser Guide Stars. Adaptive optics systems use the light from these "stars" to determine the turbulence in the atmosphere and calculate corrections one thousand times per second, commanding the thin, deformable secondary mirror of UT4 to constantly alter its shape, correcting for the distorted light.
- MUSE is not the only instrument to benefit from the Adaptive Optics Facility. Another adaptive optics system, GRAAL (GRound layer Adaptive optics Assisted by Lasers), is already in use with the infrared camera HAWK-I. This will be followed in a few years by the powerful new instrument ERIS (Enhanced Resolution Imager and Spectrograph). Together these major developments in adaptive optics are enhancing the already powerful fleet of ESO telescopes, bringing the Universe into focus.
- This new mode also constitutes a major step forward for the ESO's Extremely Large Telescope, which will need Laser Tomography to reach its science goals. These results on UT4 with the AOF will help to bring ELT's engineers and scientists closer to implementing similar adaptive optics technology on the 39 meter giant.
• July 11, 2018: New observations with ESO's Very Large Telescope show the star cluster RCW 38 in all its glory (Figure 41). This image was taken during testing of the HAWK-I (High Acuity Wide field K-band Imager) camera with the GRAAL [(Ground-layer AOM (Adaptive Optics Module) Assisted by Lasers)] adaptive optics system. It shows RCW 38 and its surrounding clouds of brightly glowing gas in exquisite detail, with dark tendrils of dust threading through the bright core of this young gathering of stars. 57)
- The central area of RCW 38 is visible here as a bright, blue-tinted region, an area inhabited by numerous very young stars and protostars that are still in the process of forming. The intense radiation pouring out from these newly born stars causes the surrounding gas to glow brightly. This is in stark contrast to the streams of cooler cosmic dust winding through the region, which glow gently in dark shades of red and orange. The contrast creates this spectacular scene — a piece of celestial artwork.
- Previous images of this region taken in optical wavelengths are strikingly different — optical images appear emptier of stars due to dust and gas blocking our view of the cluster. Observations in the infrared, however, allow us to peer through the dust that obscures the view in the optical and delve into the heart of this star cluster.
- HAWK-I is installed on Unit Telescope 4 (Yepun) of the VLT, and operates at near-infrared wavelengths. It has many scientific roles, including obtaining images of nearby galaxies or large nebulae as well as individual stars and exoplanets. GRAAL is an adaptive optics module which helps HAWK-I to produce these spectacular images. It makes use of four laser beams projected into the night sky, which act as artificial reference stars, used to correct for the effects of atmospheric turbulence — providing a sharper image.
- This image was captured as part of a series of test observations — a process known as science verification — for HAWK-I and GRAAL. These tests are an integral part of the commissioning of a new instrument on the VLT, and include a set of typical scientific observations that verify and demonstrate the capabilities of the new instrument.
- The Science Verification of HAWK-I with the GRAAL adaptive optics module was presented in an article in ESO's quarterly journal "The Messenger" entitled HAWK-I GRAAL Science Verification. 58)
- The Principal Investigator of the observing proposal which led this spectacular image was Koraljka Muzic (CENTRA, University of Lisbon, Portugal). Her collaborators were Joana Ascenso (CENTRA, University of Porto, Portugal), Amelia Bayo (University of Valparaiso, Chile), Arjan Bik (Stockholm University, Sweden), Hervé Bouy (Laboratoire d'astrophysique de Bordeaux, France), Lucas Cieza (University Diego Portales, Chile), Vincent Geers (UKATC, UK), Ray Jayawardhana (York University, Canada), Karla Peña Ramírez (University of Antofagasta, Chile), Rainer Schoedel (Instituto de Astrofísica de Andalucía, Spain), and Aleks Scholz (University of St Andrews, UK).
- The science verification team was composed of Bruno Leibundgut, Pascale Hibon, Harald Kuntschner, Cyrielle Opitom, Jerome Paufique, Monika Petr-Gotzens, Ralf Siebenmorgen, Elena Valenti and Anita Zanella, all from ESO.
Figure 41: This image shows the star cluster RCW 38, as captured by the HAWK-I infrared imager mounted on ESO's VLT (Very Large Telescope) in Chile. By gazing into infrared wavelengths, HAWK-I can examine dust-shrouded star clusters like RCW 38, providing an unparalleled view of the stars forming within. This cluster contains hundreds of young, hot, massive stars, and lies some 5500 light-years away in the constellation of Vela (The Sails), image credit:
• June 20, 2018: Each exoplanet revolves around a star, like the Earth around the Sun. This is why it is generally impossible to obtain images of an exoplanet, so dazzling is the light of its star. However, a team of astronomers, led by a researcher from the University of Geneva (UNIGE) and member of NCCR PlanetS, had the idea of detecting certain molecules that are present in the planet's atmosphere in order to make it visible, provided that these same molecules are absent from its star. Thanks to this innovative technique, the device is only sensitive to the selected molecules, making the star invisible and allowing the astronomers to observe the planet directly. 59)
Figure 42: The planet becomes visible when looking for H2O or CO molecules. However, as there is no CH4 nor NH3 in its atmosphere, it remains invisible when looking for these molecules, just as its host star which contains none of those four elements (image credit: UNIGE)
- Until now, astronomers could only very rarely directly observe the exoplanets they discovered, as they are masked by the enormous luminous intensity of their stars. Only a few planets located very far from their host stars could be distinguished on a picture, in particular thanks to the SPHERE instrument installed on the VLT (Very Large Telescope) in Chile, and similar instruments elsewhere.
- Jens Hoeijmakers, researcher at the Astronomy Department of the Observatory of the Faculty of Science of the UNIGE and member of NCCR PlanetS, wondered if it would be possible to trace the molecular composition of the planets. "By focusing on molecules present only on the studied exoplanet that are absent from its host star, our technique would effectively "erase" the star,leaving only the exoplanet," he explains.
- Erasing the star thanks to molecular spectra: To test this new technique, Jens Hoeijmakers and an international team of astronomers used archival images taken by the SINFONI(Spectrograph for INtegral Field Observations in the Near Infrared) instrument of the star beta pictoris, which is known to be orbited by a giant planet, beta pictoris b. Each pixel in these images contains the spectrum of light received by that pixel. The astronomers then compared the spectrum contained in the pixel with a spectrum corresponding to a given molecule, for example water vapor, to see if there is a correlation. If there is a correlation, it means that the molecule is present in the atmosphere of the planet.
- By applying this technique to beta pictoris b, Jens Hoeijmakers notices that the planet becomes perfectly visible when he looks for water (H2O) or carbon monoxide (CO). However, when he applies his technique to methane (CH4) and ammonia (NH3), the planet remains invisible, suggesting the absence of these molecules in the atmosphere of beta pictoris b.
- Molecules, new planetary thermometer: The host star beta pictoris remains invisible in all four situations. Indeed, this star is extremely hot and at this high temperature, these four molecules are destroyed. "This is why this technique allows us not only to detect elements on the surface of the planet, but also to sense the temperature which reigns there", explains the astronomer of UNIGE. The fact that astronomers cannot find beta pictoris b using the spectra of methane and ammonia is therefore consistent with a temperature estimated at 1700 degrees for this planet, which is too high for these molecules to exist.
- "This technique is only in its infancy", enthuses Jens Hoeijmakers. "It should change the way planets and their atmospheres are characterized. We are very excited to see what it will give on future spectrographs like ERIS (Enhanced Resolution Imager and Spectrograph) on the VLT in Chile or HARMONI (High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph) on the ELT (Extremely Large Telescope) which will be inaugurated in 2025, also in Chile," he concludes.
• April 11, 2018: The SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) instrument on ESO's VLT (Very Large Telescope) in Chile allows astronomers to suppress the brilliant light of nearby stars in order to obtain a better view of the regions surrounding them. This collection of new SPHERE images is just a sample of the wide variety of dusty discs being found around young stars (Figure 43). 60)
- These discs are wildly different in size and shape — some contain bright rings, some dark rings, and some even resemble hamburgers. They also differ dramatically in appearance depending on their orientation in the sky — from circular face-on discs to narrow discs seen almost edge-on.
- SPHERE's primary task is to discover and study giant exoplanets orbiting nearby stars using direct imaging. But the instrument is also one of the best tools in existence to obtain images of the discs around young stars — regions where planets may be forming. Studying such discs is critical to investigating the link between disc properties and the formation and presence of planets.
- Many of the young stars shown here come from a new study of T Tauri stars, a class of stars that are very young (less than 10 million years old) and vary in brightness. The discs around these stars contain gas, dust, and planetesimals — the building blocks of planets and the progenitors of planetary systems.
- These images also show what our own Solar System may have looked like in the early stages of its formation, more than four billion years ago.
- Most of the images presented were obtained as part of the DARTTS-S (Discs ARound T Tauri Stars with SPHERE) survey. The distances of the targets ranged from 230 to 550 light-years away from Earth. For comparison, the Milky Way is roughly 100 000 light-years across, so these stars are, relatively speaking, very close to Earth. But even at this distance, it is very challenging to obtain good images of the faint reflected light from discs, since they are outshone by the dazzling light of their parent stars.
- Another new SPHERE observation is the discovery of an edge-on disc around the star GSC 07396-00759, found by the SHINE (SpHere INfrared survey for Exoplanets) survey. This red star is a member of a multiple star system also included in the DARTTS-S sample but, oddly, this new disc appears to be more evolved than the gas-rich disc around the T Tauri star in the same system, although they are the same age. This puzzling difference in the evolutionary timescales of discs around two stars of the same age is another reason why astronomers are keen to find out more about discs and their characteristics.
- Astronomers have used SPHERE to obtain many other impressive images, as well as for other studies including the interaction of a planet with a disc, the orbital motions within a system, and the time evolution of a disc.
- The new results from SPHERE, along with data from other telescopes such as ALMA, are revolutionizing astronomers' understanding of the environments around young stars and the complex mechanisms of planetary formation.
Figure 43: New images from the SPHERE instrument on ESO's Very Large Telescope are revealing the dusty discs surrounding nearby young stars in greater detail than previously achieved. They show a bizarre variety of shapes, sizes and structures, including the likely effects of planets still in the process of forming (image credit: ESO/H. Avenhaus et al./E. Sissa et al./DARTT-S and SHINE collaborations) 61)
Figure 44: This spectacular image from the SPHERE instrument on ESO's Very Large Telescope shows the dusty disc around the young star IM Lupi in finer detail than ever before (image credit: ESO/H. Avenhaus et al./DARTT-S collaboration) 62)
• April 5, 2018: New images from ESO's Very Large Telescope in Chile and other telescopes reveal a rich landscape of stars and glowing clouds of gas in one of our closest neighbouring galaxies, the Small Magellanic Cloud. The pictures have allowed astronomers to identify an elusive stellar corpse buried among filaments of gas left behind by a 2000-year-old supernova explosion. The MUSE instrument was used to establish where this elusive object is hiding, and existing Chandra X-ray Observatory data confirmed its identity as an isolated neutron star. 63) 64)
- Spectacular new pictures, created from images from both ground- and space-based telescopes (Figure 45), tell the story of the hunt for an elusive missing object hidden amid a complex tangle of gaseous filaments in the Small Magellanic Cloud, about 200 000 light-years from Earth.
- New data from the MUSE instrument on ESO's Very Large Telescope in Chile has revealed a remarkable ring of gas in a system called 1E 0102.2-7219, expanding slowly within the depths of numerous other fast-moving filaments of gas and dust left behind after a supernova explosion. This discovery allowed a team led by Frédéric Vogt, an ESO Fellow in Chile, to track down the first ever isolated neutron star with low magnetic field located beyond our own Milky Way galaxy.
- The team noticed that the ring was centered on an X-ray source that had been noted years before and designated p1. The nature of this source had remained a mystery. In particular, it was not clear whether p1 actually lies inside the remnant or behind it. It was only when the ring of gas — which includes both neon and oxygen — was observed with MUSE that the science team noticed it perfectly circled p1. The coincidence was too great, and they realized that p1 must lie within the supernova remnant itself. Once p1's location was known, the team used existing X-ray observations of this target from the Chandra X-ray Observatory to determine that it must be an isolated neutron star, with a low magnetic field.
- In the words of Frédéric Vogt: "If you look for a point source, it doesn't get much better than when the Universe quite literally draws a circle around it to show you where to look."
- When massive stars explode as supernovae, they leave behind a curdled web of hot gas and dust, known as a supernova remnant. These turbulent structures are key to the redistribution of the heavier elements — which are cooked up by massive stars as they live and die — into the interstellar medium, where they eventually form new stars and planets.
- Typically barely ten kilometers across, yet weighing more than our Sun, isolated neutron stars with low magnetic fields are thought to be abundant across the Universe, but they are very hard to find because they only shine at X-ray wavelengths [Highly-magnetic spinning neutron stars are called pulsars. They emit strongly at radio and other wavelengths and are easier to find, but they are only a small fraction of all the neutron stars predicted to exist]. The fact that the confirmation of p1 as an isolated neutron star was enabled by optical observations is thus particularly exciting.
- Co-author Liz Bartlett, another ESO Fellow in Chile, sums up this discovery: "This is the first object of its kind to be confirmed beyond the Milky Way, made possible using MUSE as a guidance tool. We think that this could open up new channels of discovery and study for these elusive stellar remains."
Figure 45: An isolated neutron star in the Small Magellanic Cloud. The image combines data from the MUSE instrument on ESO's Very Large Telescope in Chile and the orbiting the NASA/ESA Hubble Space Telescope and NASA Chandra X-Ray Observatory (image credit: ESO)
Legend to Figure 45: The reddish background image comes from the NASA/ESA Hubble Space Telescope and reveals the wisps of gas forming the supernova remnant 1E 0102.2-7219 in green. The red ring with a dark center is from the MUSE instrument on ESO's Very Large Telescope and the blue and purple images are from the NASA Chandra X-Ray Observatory. The blue spot at the center of the red ring is an isolated neutron star with a weak magnetic field, the first identified outside the Milky Way.
• March 5, 2018: First Light of the MATISSE Interferometer at ESO's Very Large Telescope in Chile. After 12 years of design and development in Europe, the MATISSE interferometry instrument has been installed during the last 3 months at ESO's VLT (Very Large Telescope). MATISSE combines four of the VLT telescopes to obtain an interferometer with an extremely high spatial resolution. This instrument allows astronomers to study the environment of young stars, the surfaces of stars and Active Galactic Nuclei in the mid-infrared wavelength range. In February 2018, this new, powerful and technically challenging instrument successfully achieved ‘First Light'. This achievement consummates the decade-long efforts of a large number of engineers and astronomers in France, Germany and in the Netherlands, including the infrared interferometry research group at the Max Planck Institute for Radio Astronomy in Bonn, Germany. 65) 66)
- The initial MATISSE observations of the red supergiant star Betelgeuse, which is expected to explode as a supernova in a few hundred thousand years, showed that it still has secrets to reveal. The new observations show evidence that the star appears to have a different size when seen at different wavelengths. Such data will allow astronomers to further study the huge star's surroundings and how it is shedding material into space.
- The principal investigator of MATISSE, Bruno Lopez (Observatoire de la Côte d'Azur (OCA), Nice, France), explains its unique power: "Single telescopes can achieve image sharpness that is limited by the size of their mirrors. To obtain even higher resolution, we combine — or interfere — the light from four different VLT telescopes. Doing this enables MATISSE to deliver the sharpest images of any telescope ever in the 3–13 µm wavelength range, where it will complement the James Webb Space Telescope's future observations from space."
- Thomas Henning, director at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, and MATISSE co-principal investigator, comments: "By looking at the inner regions of protoplanetary discs with MATISSE, we hope to learn the origin of the various minerals contained in these discs — minerals that will later go on to form the solid cores of planets like the Earth."
- Walter Jaffe, the project scientist and co-principal investigator from University of Leiden in the Netherlands, and Gerd Weigelt, co-principal investigator from the Max Planck Institute for Radio Astronomy (MPIfR), Bonn, Germany, add: "MATISSE will give us dramatic images of planet-forming regions, multiple stars, and, when working with the VLT Unit Telescopes, also the dusty discs feeding supermassive black holes. We hope also to observe details of exotic objects in our Solar System, such as volcanoes on Io, and the atmospheres of giant exoplanets."
- MATISSE's first light marks a big step forward in the scope of current optical/infrared interferometers and will allow astronomers to obtain interferometric images with finer detail over a wider wavelength range than currently possible. MATISSE will also complement the instruments planned for ESO's upcoming ELT (Extremely Large Telescope), in particular METIS (Mid-infrared ELT Imager and Spectrograph). MATISSE will observe brighter objects than METIS, but with higher spatial resolution.
- Andreas Glindemann, MATISSE project manager at ESO, concludes: "Making MATISSE a reality has involved the work of many people over many years and it is wonderful to see the instrument working so well. We are looking forward to the exciting science to come!"
Figure 46: Four-telescope interferogram of the star Sirius recorded at "First light" observations on 18 February 2018 with VLTI-MATISSE. This image is a colorized version of the interferogram recorded at infrared wavelengths. Blue color corresponds to short infrared wavelengths, red corresponds to long wavelengths. The colors illustrate the changing wavelengths of the data. Interferograms are the raw data required for reconstructing high-resolution images of astronomical objects (image credit: ESO/MATISSE Consortium)
• February 17, 2018: It seems nothing can escape the inexorable spread of light pollution — not even the giant telescopes probing the heavens above northern Chile, a region whose pristine dark skies, long considered a paradise for astronomers, are under increasing threat. 67)
- The Atacama desert, 1,200 km north of the capital Santiago, provides ideal conditions where astronomers study the stars in darkness so profound they appear like diamonds on velvet. - Scientists estimate that by 2020, Chile — a critically important country for optical and radio astronomy — will host 70 percent of the globe's astronomical infrastructure.
- But the ever-expanding use of cheap LED (Light-Emitting Diode) lighting in the booming South American country is starting to concern astronomers desperately trying to safeguard some of the world's darkest skies.
- "Unfortunately, as we have more and more white lights, the deterioration of the skies has increased by up to 30 percent compared to the end of the last decade," said scientist Pedro Sanhueza.
- Chile takes the problem of light pollution so seriously that Sanhueza heads up an organization called the Office for the Protection of Quality of the Sky (OPCC).
Figure 47: Moonshine photo of the VLT (Very Large Telescope) observatory on the Cerro Paranal mountain in the Atacama Desert of northern Chile (image credit: ESO)
- Its main task is to make the people of northern Chile aware of the particularly high night-sky quality and the negative impacts of light pollution.
- Sanhueza says that though the quality is good, the sky over northern Chile is becoming "an area of risk," threatening the profound nocturnal darkness required for the study of phenomena such as solar flares, planetary nebulae, black holes and supernovas.
- Fueling the threat, he adds, are communities such as Antofagasta, Coquimbo and La Serena, where LED lights are increasingly used in homes, streetlights, store signs and billboards.
- A study published in December in the journal Science Advances has shown that global lighting has increased in both quantity and intensity by about 2 percent per year from 2012 to 2016.
- Urban boom: At the Paranal Observatory deep in the Atacama desert, which houses the European Southern Observatory's Very Large Telescope array — consisting of four telescopes — staff are doing all they can to limit light leaking out into the atmosphere.
- After sunset, vehicles travelling around the observatory are prohibited anything but their parking lights. Flashlights, if needed, are turned to the ground.
- Astronomers' residences at the base — 2,635 meters above sea level — are dimly-lit, to avoid interfering with observation of the galaxies above.
- But the urban boom has been an unstoppable reality for 20 years in northern Chile, where cities have felt the economic effects of the boom in copper, of which the South American country is the world's largest producer.
- Halos of light above towns are easily visible from observatories within a 150 km radius.
- "We have measured the impact of this and we have already experienced difficulties making observations at 20 degrees above the horizon. That's going to increase a lot and will prevent us from studying the furthest stars," said Chris Smith, head of the observatory at Tololo, some 80 km from the town of La Serena.
- Hazardous to health: Urban growth has gone hand in hand with huge infrastructure projects to extract copper and even the construction of a brightly-lit highway through the Atacama itself.
- Smith is calling for more education in schools about the sustainable use of light, the need to use "warmer" sources of lighting that are less polluting, and to avoid turning them towards the sky. "We are already seeing a big level of impact and we need to control it now," Smith, an American astronomer, says — adding there can be "no question of shutting observatories down."
- However, this is what many fear could ultimately happen to the Mount Palomar observatory in California, which has had to drastically cut down its activities because of the light pollution from Los Angeles.
- The Chilean government in 2012 approved a new lighting standard designed to cut down on pollution, but nevertheless, scientists acknowledge that it's an uphill battle.
• February 13, 2018: This ghostly image features a distant and pulsating red giant star known as R Sculptoris. Situated 1,200 light-years away in the constellation of Sculptor, R Sculptoris is something known as a carbon-rich AGB (Asymptotic Giant Branch) star, meaning that it is nearing the end of its life. At this stage, low- and intermediate-mass stars cool off, create extended atmospheres, and lose a lot of their mass — they are on their way to becoming spectacular planetary nebulae. 68) 69) 70)
- While the basics of this mass-loss process are understood, astronomers are still investigating how it begins near the surface of the star. The amount of mass lost by a star actually has huge implications for its stellar evolution, altering its future, and leading to different types of planetary nebulae. As AGB stars end their lives as planetary nebulae, they produce a vast range of elements — including 50% of elements heavier than iron — which are then released into the Universe and used to make new stars, planets, moons, and eventually the building blocks of life.
- One particularly intriguing feature of R Sculptoris is its dominant bright spot, which looks to be two or three times brighter than the other regions. The astronomers that captured this wonderful image, using ESO's VLTI (Very Large Telescope Interferometer ), have concluded that R Sculptoris is surrounded by giant "clumps" of stellar dust that are peeling away from the shedding star. This bright spot is, in fact, a region around the star with little to no dust, allowing us to look deeper into the stellar surface.
Figure 48: Image of the pulsating red giant star R Sculptoris, based on observations made with the VLTI Paranal Observatory under program IDs 090.D-0136, 093.D-0015, 096.D-0720 (image credit, ESO, Research Team)
• January 30, 2018: ESO's VLT (Very Large Telescope) now has a second instrument working with the powerful AOF (Adaptive Optics Facility). The infrared instrument HAWK-I (High Acuity Wide-field K-band Imager) is now also benefiting from sharper images and shorter exposure times. This follows the successful integration of the AOF with MUSE (Multi Unit Spectroscopic Explorer). 71)
- The AOF is a long-term project that is nearing completion on ESO's VLT. It provides adaptive optics correction for all the instruments attached to one of the VLT Unit Telescopes (UT4, also known as Yepun).
- Adaptive optics works to compensate for the blurring effect of the Earth's atmosphere. This upgrade now enables HAWK-I to obtain sharper images, needing shorter exposure times than before to obtain similar results. By using the AOF, astronomers can now get good image quality with HAWK-I, even when the weather conditions are not perfect.
- Following a series of tests of the new system, the commissioning team of astronomers and engineers were rewarded with a series of spectacular images, including one of the Tarantula Nebula star-forming region in the Large Magellanic Cloud.
- The AOF, which made these observations possible, is composed of many parts working together. These include the Four Laser Guide Star Facility (4LGSF) and the UT4's very thin deformable secondary mirror, which is able to change its shape. The 4LGSF shines four 22 W laser beams into the sky to make sodium atoms in the upper atmosphere glow as bright points of light, forming artificial guide stars.
- Sensors in the adaptive optics module GRAAL (GRound layer Adaptive optics Assisted by Lasers) use these artificial guide stars to determine the atmospheric conditions. One thousand times per second, the AOF system calculates the correction that must be applied to the telescope's deformable secondary mirror to compensate for the atmospheric disturbance.
- GRAAL corrects for the turbulence in the layer of atmosphere up to about 500 m above the telescope — the "ground layer". Depending on the conditions, atmospheric turbulence occurs at all altitudes, but studies have shown that the largest fraction of the disturbance occurs in the ground layer of the atmosphere.
- The corrections applied by the AOF rapidly and continuously improve the image quality by concentrating the light to form sharper images, allowing HAWK-I to resolve finer details and detect fainter stars than previously possible.
- MUSE and HAWK-I are not the only instruments that will benefit from the AOF; in the future, the new instrument ERIS (Enhanced Resolution Imager and Spectrograph) will be installed on the VLT. The AOF is also a pathfinder for adaptive optics on ESO's ELT (Extremely Large Telescope).
Figure 49: This image of the dramatic star formation region 30 Doradus, also known as the Tarantula Nebula, was created from a mosaic of images taken using the HAWK-I instrument working with the Adaptive optics Facility of ESO's Very Large Telescope in Chile. The stars are significantly sharper than the same image without adaptive optics being used, and fainter stars can be seen (image credit: ESO) 72)
• January 17, 2018: Globular star clusters are huge spheres of tens of thousands of stars that orbit most galaxies. They are among the oldest known stellar systems in the Universe and date back to near the beginning of galaxy growth and evolution. More than 150 are currently known to belong to the Milky Way. 73)
- One particular cluster, called NGC 3201 and situated in the southern constellation of Vela (The Sails), has now been studied using the MUSE instrument on ESO's Very Large Telescope in Chile. An international team of astronomers has found that one of the stars in NGC 3201 is behaving very oddly — it is being flung backwards and forwards at speeds of several hundred thousand kilometers per hour, with the pattern repeating every 167 days.
Figure 50: Artist's impression of the black hole binary system in NGC 3201. Astronomers using ESO's MUSE instrument on the Very Large Telescope in Chile have discovered a star in the cluster NGC 3201 that is behaving very strangely. It appears to be orbiting an invisible black hole with about four times the mass of the Sun — the first such inactive stellar-mass black hole found in a globular cluster. This important discovery impacts on our understanding of the formation of these star clusters, black holes, and the origins of gravitational wave events. This artist's impression shows how the star and its massive but invisible black hole companion may look, in the rich heart of the globular star cluster (image credit: ESO/L. Calçada/spaceengine.org)
- Lead author Benjamin Giesers (Georg-August-Universität Göttingen, Germany) was intrigued by the star's behavior: "It was orbiting something that was completely invisible, which had a mass more than four times the Sun — this could only be a black hole! The first one found in a globular cluster by directly observing its gravitational pull." 74)
- The relationship between black holes and globular clusters is an important but mysterious one. Because of their large masses and great ages, these clusters are thought to have produced a large number of stellar-mass black holes — created as massive stars within them exploded and collapsed over the long lifetime of the cluster.
- ESO's MUSE instrument provides astronomers with a unique ability to measure the motions of thousands of far away stars at the same time. With this new finding, the team have for the first time been able to detect an inactive black hole at the heart of a globular cluster — one that is not currently swallowing matter and is not surrounded by a glowing disc of gas. They could estimate the black hole's mass through the movements of a star caught up in its enormous gravitational pull.
- From its observed properties the star was determined to be about 0.8 times the mass of our Sun, and the mass of its mysterious counterpart was calculated at around 4.36 times the Sun's mass — almost certainly a black hole.
- Recent detections of radio and X-ray sources in globular clusters, as well as the 2016 detection of gravitational-wave signals produced by the merging of two stellar-mass black holes, suggest that these relatively small black holes may be more common in globular clusters than previously thought.
- Giesers concludes: "Until recently, it was assumed that almost all black holes would disappear from globular clusters after a short time and that systems like this should not even exist! But clearly this is not the case — our discovery is the first direct detection of the gravitational effects of a stellar-mass black hole in a globular cluster. This finding helps in understanding the formation of globular clusters and the evolution of black holes and binary systems — vital in the context of understanding gravitational wave sources."
• December 13, 2017: The OmegaCAM camera on ESO's VLT Survey Telescope has captured this glittering view of the stellar nursery called Sharpless 29 (Figure 51). Many astronomical phenomena can be seen in this giant image, including cosmic dust and gas clouds that reflect, absorb, and re-emit the light of hot young stars within the nebula. 75)
- The region of sky pictured is listed in the Sparpless Catalog of H II regions: interstellar clouds of ionized gas, rife with star formation. Also known as Sh 2-29, Sharpless 29 is located about 5500 light-years away in the constellation of Sagittarius (The Archer), next door to the larger Lagoon Nebula. It contains many astronomical wonders, including the highly active star formation site of NGC 6559, the nebula at the center of the image.
- This central nebula is Sharpless 29's most striking feature. Though just a few light-years across, it showcases the havoc that stars can wreak when they form within an interstellar cloud. The hot young stars in this image are no more than two million years old and are blasting out streams of high-energy radiation. This energy heats up the surrounding dust and gas, while their stellar winds dramatically erode and sculpt their birthplace. In fact, the nebula contains a prominent cavity that was carved out by an energetic binary star system. This cavity is expanding, causing the interstellar material to pile up and create the reddish arc-shaped border.
- When interstellar dust and gas are bombarded with ultraviolet light from hot young stars, the energy causes them to shine brilliantly. The diffuse red glow permeating this image comes from the emission of hydrogen gas, while the shimmering blue light is caused by reflection and scattering off small dust particles. As well as emission and reflection, absorption takes place in this region. Patches of dust block out the light as it travels towards us, preventing us from seeing the stars behind it, and smaller tendrils of dust create the dark filamentary structures within the clouds.
- The rich and diverse environment of Sharpless 29 offers astronomers a smorgasbord of physical properties to study. The triggered formation of stars, the influence of the young stars upon dust and gas, and the disturbance of magnetic fields can all be observed and examined in this single area.
- But young, massive stars live fast and die young. They will eventually explosively end their lives in a supernova, leaving behind rich debris of gas and dust. In tens of millions of years, this will be swept away and only an open cluster of stars will remain.
- Sharpless 29 was observed with ESO's OmegaCAM on the VLT Survey Telescope (VST) at Cerro Paranal in Chile. OmegaCAM produces images that cover an area of sky more than 300 times greater than the largest field of view imager of the NASA/ESA Hubble Space Telescope, and can observe over a wide range of wavelengths from the ultraviolet to the infrared. Its hallmark feature is its ability to capture the very red spectral line H-alpha, created when the electron inside a hydrogen atom loses energy, a prominent occurrence in a nebula like Sharpless 29.
Figure 51: The OmegaCAM camera on ESO's VLT Survey Telescope has captured this glittering view of the stellar nursery called Sharpless 29. Many astronomical phenomena can be seen in this giant image, including cosmic dust and gas clouds that reflect, absorb, and re-emit the light of hot young stars within the nebula (image credit: ESO)
• August 23, 2017: To the unaided eye the famous, bright star Antares shines with a strong red tint in the heart of the constellation of Scorpius (The Scorpion). It is a huge and comparatively cool red supergiant star in the late stages of its life, on the way to becoming a supernova. 76)
- A team of astronomers, led by Keiichi Ohnaka, of the Universidad Católica del Norte in Chile, has now used ESO's Very Large Telescope Interferometer (VLTI) at the Paranal Observatory in Chile to map Antares's surface and to measure the motions of the surface material. This is the best image of the surface and atmosphere of any star other than the Sun.
- The VLTI is a unique facility that can combine the light from up to four telescopes, either the 8.2 meter Unit Telescopes, or the smaller Auxiliary Telescopes, to create a virtual telescope equivalent to a single mirror up to 200 m across. This allows it to resolve fine details far beyond what can be seen with a single telescope alone.
- "How stars like Antares lose mass so quickly in the final phase of their evolution has been a problem for over half a century," said Keiichi Ohnaka, who is also the lead author of the paper. "The VLTI is the only facility that can directly measure the gas motions in the extended atmosphere of Antares — a crucial step towards clarifying this problem. The next challenge is to identify what's driving the turbulent motions."
- Using the new results the team has created the first two-dimensional velocity map of the atmosphere of a star other than the Sun. They did this using the VLTI with three of the Auxiliary Telescopes and an instrument called AMBER to make separate images of the surface of Antares over a small range of infrared wavelengths. The team then used these data to calculate the difference between the speed of the atmospheric gas at different positions on the star and the average speed over the entire star. This resulted in a map of the relative speed of the atmospheric gas across the entire disc of Antares — the first ever created for a star other than the Sun.
- The astronomers found turbulent, low-density gas much further from the star than predicted, and concluded that the movement could not result from convection, that is, from large-scale movement of matter which transfers energy from the core to the outer atmosphere of many stars. They reason that a new, currently unknown, process may be needed to explain these movements in the extended atmospheres of red supergiants like Antares.
- "In the future, this observing technique can be applied to different types of stars to study their surfaces and atmospheres in unprecedented detail. This has been limited to just the Sun up to now," concludes Ohnaka. "Our work brings stellar astrophysics to a new dimension and opens an entirely new window to observe stars."
Figure 52: Using ESO's VLTI astronomers have constructed the most detailed image ever of a star — the red supergiant star Antares. They have also made the first map of the velocities of material in the atmosphere of a star other than the Sun, revealing unexpected turbulence in Antares's huge extended atmosphere. The results were published in the journal Nature. 77)
• May 23, 2013: This new picture celebrates an important anniversary for the VLT (Very Large Telescope) – it is fifteen years since the first light on the first of its four Unit Telescopes, on 25 May 1998. Since then the four original giant telescopes have been joined by the four small Auxiliary Telescopes that form part of the VLT Interferometer (VLTI). The VLT is one of the most powerful and productive ground-based astronomical facilities in existence. In 2012 more than 600 refereed scientific papers based on data from the VLT and VLTI were published (ann13009). 78)
- Interstellar clouds of dust and gas are the nurseries where new stars are born and grow. The new picture shows one of them, IC 2944, which appears as the softly glowing pink background . This image is the sharpest view of the object ever taken from the ground. The cloud lies about 6500 light-years away in the southern constellation of Centaurus (The Centaur). This part of the sky is home to many other similar nebulae that are scrutinized by astronomers to study the mechanisms of star formation.
- Emission nebulae like IC 2944 are composed mostly of hydrogen gas that glows in a distinctive shade of red, due to the intense radiation from the many brilliant newborn stars. Clearly revealed against this bright backdrop are mysterious dark clots of opaque dust, cold clouds known as Bok globules. They are named after the Dutch-American astronomer Bart Bok, who first drew attention to them in the 1940s as possible sites of star formation. This particular set is nicknamed the Thackeray Globules (they were discovered from South Africa by the English astronomer A. David Thackeray in 1950).
- Larger Bok globules in quieter locations often collapse to form new stars but the ones in this picture are under fierce bombardment from the ultraviolet radiation from nearby hot young stars. They are both being eroded away and also fragmenting, rather like lumps of butter dropped into a hot frying pan. It is likely that Thackeray's Globules will be destroyed before they can collapse and form stars.
- Bok globules are not easy to study. As they are opaque to visible light it is difficult for astronomers to observe their inner workings, and so other tools are needed to unveil their secrets — observations in the infrared or in the submillimeter parts of the spectrum, for example, where the dust clouds, only a few degrees over absolute zero, appear bright. Such studies of the Thackeray globules have confirmed that there is no current star formation within them.
Figure 53: With this new view of a spectacular stellar nursery ESO is celebrating 15 years of the Very Large Telescope — the world's most advanced optical instrument. This picture reveals thick clumps of dust silhouetted against the pink glowing gas cloud known to astronomers as IC 2944. These opaque blobs resemble drops of ink floating in a strawberry cocktail, their whimsical shapes sculpted by powerful radiation coming from the nearby brilliant young stars (image credit: ESO)
• August 2011: The Eyes are about 50 million light-years away in the constellation of Virgo (The Virgin) and are some 100,000 light-years apart. The nickname comes from the apparent similarity between the cores of this pair of galaxies — two white ovals that resemble a pair of eyes glowing in the dark when seen in a moderate-sized telescope. 79)
- But although the centers of these two galaxies look similar, their outskirts could not be more different. The galaxy in the lower right, known as NGC 4435, is compact and seems to be almost devoid of gas and dust. In contrast, in the large galaxy in the upper left (NGC 4438) a lane of obscuring dust is visible just below its nucleus, young stars can be seen left of its center, and gas extends at least up to the edges of the image.
- The contents of NGC 4438 have been stripped out by a violent process: a collision with another galaxy. This clash has distorted the galaxy's spiral shape, much as could happen to the Milky Way when it collides with its neighboring galaxy Andromeda in three or four billion years.
- NGC 4435 could be the culprit. Some astronomers believe that the damage caused to NGC 4438 resulted from an approach between the two galaxies to within about 16,000 light-years that happened some 100 million years ago. But while the larger galaxy was damaged, the smaller one was significantly more affected by the collision. Gravitational tides from this clash are probably responsible for ripping away the contents of NGC 4438, and for reducing NGC 4435's mass and removing most of its gas and dust.
- Another possibility is that the giant elliptical galaxy Messier 86, further away from The Eyes and not visible in this image, was responsible for the damage caused to NGC 4438. Recent observations have found filaments of ionized hydrogen gas connecting the two large galaxies, indicating that they may have collided in the past.
- The elliptical galaxy Messier 86 and The Eyes belong to the Virgo Cluster, a very rich grouping of galaxies. In such close quarters, galaxy collisions are fairly frequent, so perhaps NGC 4438 suffered from encounters with both NGC 4435 and Messier 86.
- This picture is the first to be produced as part of the ESO Cosmic Gems program (Figure 54). This is a new initiative to produce astronomical images for educational and public outreach purposes. The program mainly makes use of time when the sky conditions are not suitable for science observations to take pictures of interesting, intriguing or visually attractive objects. The data are also made available to professional astronomers through ESO's science archive.
- In this case, although there were some clouds, the atmosphere was exceptionally stable, which allowed very sharp details to be revealed in this image taken using the VLT's FORS2 (FOcal Reducer and low dispersion Spectrograph) instrument (installed on the VLT's Unit Telescope 1). Light passing through two different filters was used: red (colored red) and green-yellow (colored blue), and the exposure times were 1800 seconds and 1980 seconds, respectively.
Figure 54: ESO's Very Large Telescope has taken a striking image of a beautiful yet peculiar pair of galaxies nicknamed The Eyes. The larger of these, NGC 4438, was once a spiral galaxy but has become badly deformed by collisions with other galaxies in the last few hundred million years. This picture is the first to come out of ESO's Cosmic Gems program, an initiative in which ESO has granted dedicated observing time for outreach purposes (image credit: ESO)
• March 13, 2002: One of the most fundamental tasks of modern astrophysics is the study of the evolution of the Universe . This is a daunting undertaking that requires extensive observations of large samples of objects in order to produce reasonably detailed maps of the distribution of galaxies in the Universe and to perform statistical analysis. Much effort is now being put into mapping the relatively nearby space and thereby to learn how the Universe looks today . But to study its evolution, we must compare this with how it looked when it still was young . This is possible, because astronomers can "look back in time" by studying remote objects - the larger their distance, the longer the light we now observe has been underway to us, and the longer is thus the corresponding "look-back time." This may sound easy, but it is not. Very distant objects are very dim and can only be observed with large telescopes. Looking at one object at a time would make such a study extremely time-consuming and, in practical terms, impossible. To do it anyhow, we need the largest possible telescope with a highly specialized, exceedingly sensitive instrument that is able to observe a very large number of (faint) objects in the remote universe simultaneously. 80) 81)
- VIMOS (VLT VIsible Multi-Object Spectrograph) is such an instrument. It can obtain many hundreds of spectra of individual galaxies in the shortest possible time; in fact, in one special observing mode, up to 6400 spectra of the galaxies in a remote cluster during a single exposure, augmenting the data gathering power of the telescope by the same proportion. This marvelous science machine has just been installed at the 8.2-m MELIPAL telescope, the third unit of the VLT (Very Large Telescope) at the ESO Paranal Observatory. A main task will be to carry out 3-dimensional mapping of the distant Universe from which we can learn its large-scale structure.
- "First light" was achieved on February 26, 2002, and a first series of test observations has successfully demonstrated the huge potential of this amazing facility. Much work on VIMOS is still ahead during the coming months in order to put into full operation and fine-tune the most efficient "galaxy cruncher" in the world. VIMOS is the outcome of a fruitful collaboration between ESO and several research institutes in France and Italy, under the responsibility of the Laboratoire d'Astrophysique de Marseille (CNRS, France). The other partners in the "VIRMOS Consortium" are the Laboratoire d'Astrophysique de Toulouse, Observatoire Midi-Pyrénées, and Observatoire de Haute-Provence in France, and Istituto di Radioastronomia (Bologna), Istituto di Fisica Cosmica e Tecnologie Relative (Milano), Osservatorio Astronomico di Bologna, Osservatorio Astronomico di Brera (Milano) and Osservatorio Astronomico di Capodimonte (Naples) in Italy.
Figure 55: The Crab Nebula (Messier 1), as observed by VIMOS. This well-known object is the remnant of a stellar explosion in the year 1054 (image credit: ESO)
Legend to Figure 55: The image is a composite VRI image obtained on March 4, 2002. The individual exposures lasted 180 seconds; image quality 0.7 arcsec FWHM; field 7 x 7 arcmin2; North is up and East is left.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).