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VLT (Very Large Telescope) of ESO

Background    ESPRESSO    Mission Status   FLAMES   MATISSE   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.


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. 2)

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. 3)

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


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. 4)

- 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 2: 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. 5)

- 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 3: 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. 6) 7)

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 4: 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 4. The spectral format covered by the blue and the red chips as well as the shape of the pseudo slit are illustrated by Figure 5.


Figure 5: 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. 8) 9)

- 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 6: 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. 10)

- 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 7: 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. 11)

- 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 8: 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. 12)

- 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 9: 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 10 and 11. 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. 13) 14)


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 10: 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 11: 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 12. 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 12: 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. 15)

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 13):


Figure 13: 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. 16)

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: 17)

• 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.



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. 18) 19) 20)

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: 21)

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. 21):

• 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 14: 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 15: The very complex instrument MATISSE during installation at Paranal Observatory (image credit: ESO, P. Horálek) 22)



Status and sample Observations with VLT

• 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. 23) 24)

- 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 16: 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. 25)

- 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 17: 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. 26) 27) 28)

- 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 18: 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). 29)

- 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 19: 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) 30)

• 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 20). 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. 31)

- 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 20: 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. 32)

- 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 21: 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. 33)

• 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). 34)

- 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 22: 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. 35)

- 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 23). 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 23: 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. 36) 37)

- 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 24: 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 24: 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 (

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