Minimize VLT (Very Large Telescope)

VLT (Very Large Telescope) of ESO on Cerro Paranal

ESPRESSO   Hawk-I   FLAMES   MATISSE   GRAVITY   MUSE   Mission Status   References

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)


Some Background:

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

Llano de Chajnantor, which hosts the APEX (Atacama Pathfinder Experiment) submillimeter telescope and where ALMA (Atacama Large Millimeter/submillimeter Array), 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.





Location (Chile)


ESO 3.6 m telescope – hosting HARPS

ESO 3.6 m

3.57 m

optical and infrared

La Silla


MPG/ESO 2.2 m telescope


2.20 m

optical and infrared

La Silla


New Technology Telescope


3.58 m

optical and infrared

La Silla


Very Large Telescope


4 x 8.2 m, 4 x 1.8 m

optical and mid-infrared



Atacama Pathfinder Experiment


12 m

mm/sub-mm wavelength



Visible and Infrared Survey Telescope for Astronomy


4.1 m

near-infrared, survey



VLT Survey Telescope


2.6 m

optical, survey



Atacama Large Millimeter/submillimeter Array


50 x 12 m, 12 x 7 m
4 x 12 m

mm/sub-mm interferometer



Extremely Large Telescope


39.3 m

optical to mid-infrared

Cerro Amazones


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).

Instrument Description:

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.

A sketch of the optical layout is shown in Figure 6. The spectral format covered by the blue and the red chips as well as the shape of the pseudo slit are illustrated by Figure 7.


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)


106 mas/pixel

FOV (Field of View)

7.5 arcmin x 7.5 arcmin

Image quality (80% EE)

<0.2 arcsec


<0.3% across the field

Optics throughput (w/o detector)



4 Broad band, 6 Narrow band


four 2 k x 2 k Hawaii2RG arrays

Detector quantum efficiency


Detector temperature

75 K ± 1 mK

Read noise

~5 e- for integration times >15 s

Instrument background

~0.10-0.15 e-/s

Instrument temperature

<140 K

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
to 0.106'' on the sky). The detectors are separated by gaps of about 15'' as shown in Figure 14. The figure also shows the naming convention of the four detectors. The images of the four detectors are stored together in a single FITS file as four extensions. Note that quadrants 1, 2, 3, 4 are usually, but not necessarily, stored in extensions 1, 2, 4, 3 of the HAWK-I FITS file.

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)


No of columns or rows vignetted >10%

Maximum vignetting













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.
Note: MUSE entered a new era in 2017 with the advanced capabilities of the AOF (Adaptive Optics Facility). 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 metres above the Paranal summit, a height free of the effects of the most turbulent layers of the atmosphere, giving much clearer images than before.

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.

Instrument capabilities:

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.

Instrument mode


Direct Imaging (E2V)

U=25.9, B=27.6, V=27.3, R=26.6, I=25.8

Direct Imaging (MIT)

U=24.5, B=27.1, V=27.0, R=26.7, I=25.7, z=24.7

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.

Instrument mode

Rs = λ/Δλ


Longslit Spectroscopy [1]



MOS - movable slits [2]



MXU - exchangeable masks






Table 5: FORS2 spectroscopic modes

Legend to Table 5: [1] In longslit spectroscopy the slit is chosen out of a set of 9 slits with fixed width between 0.3'' and 2.5''. [2] 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)

• FORS1 and FORS2 broke several distance records: the most distant gamma-ray burst (eso0034), the most distant group of galaxies (eso0212), the most distant galaxy (eso0314)

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.

MATISSE Consortium:

- 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).

Instrument description:

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 instrument

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.

Instrument Description:

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)