From ESOblog (EU): “VLTI-your (birth) story goes on!”

From ESOblog (EU)


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European Southern Observatory [Observatoire européen austral][Europäische Südsternwarte] (EU) (CL)

5 November 2021
On the Ground

Ready to rewind the tape once more? After a one-week break, let’s go back to the 29th of October 2001 on Cerro Paranal in the Atacama Desert, when two of ESO’s 8.2-m telescopes were about to be linked for the first time with the Very Large Telescope Interferometer (VLTI) [below]. To celebrate the twentieth anniversary of those historical moments, last week we heard the memories of two scientists who experienced that night in person. Today, we close the circle and listen to two other scientists in the extraordinary team that made this possible.

If you missed our previous post [ ] in this two-part series you may be wondering why we want to combine the light of several telescopes. The larger a telescope is, the finer the details it can discern. By linking two or more telescopes we can create a giant “virtual” telescope as large as the separation between them. This technique, called interferometry, allows us to resolve details much smaller than what is possible with each individual telescope alone.

When lightwaves from two different telescopes pointing at the same target meet, they interfere, creating bright and dark stripes. These so-called “fringes” contain information about the size of the object being observed; linking several telescopes at different separations and orientations is necessary to reconstruct an image of said object. This is easier said than done, though, and recording fringes with ESO’s 8.2-m Unit Telescopes (UTs) took years of preparation…

Learning from ancient sailors

“We could see the first fringes with the two UTs within the first few hours of the night,” remembers Philippe Gitton, an optical engineer at ESO. “It was quick and may have looked easy, but it was the result of many, many years of preparatory activities.”

A key preparation step took place a few months earlier: the observation of the first fringes with two small test telescopes called siderostats, which was by no means a walk in the park.

One of the two Siderostats, 0.3-metre fixed telescopes used to feed the VLTI instrument VINCI during test and early science phases.


VINCI was a special test instrument installed on ESO’s Very Large Telescope Interferometer (VLTI) at the Paranal Observatory. It was used during the first light and subsequent test phase of the VLTI, when light from celestial objects hit the telescopes for the first time.

The VLTI is a complex system. To ensure that the many devices involved worked properly together and could produce meaningful data, the VLTI went through a period of extensive testing and verification. This testing period included using VINCI to capture the very first light the telescopes saw, before the arrival of the official instruments AMBER and MIDI.

VINCI was able to combine the light of two of the eight telescopes comprising the VLT and accurately measure the obtained interference fringes. The main components of this high-tech instrument are aptly named MONA, a system that combines the light beams from two telescopes by means of optical fibres, and LISA, the infrared camera.

On 17 March 2001, the VLT Interferometer was used for the first time to carry out an astronomical observation, using siderostats — a type of heliostat designed to follow stars — instead of telescopes for this test. The target was Sirius (Alpha Canis Majoris), which is the brightest star in the sky, although it is in reality a binary star system.

Andreas Glindemann, head of the VLTI programme at the time, commented: “The tension was intense when starlight was guided for the first time from the primary mirror of the siderostats, through the light ducts, the tunnel and the beam combination laboratory to the detector of VINCI. And, after a few nights, the result was spectacular. The very first result, the fringe pattern of Sirius… This was a joyful moment and the champagne corks were popping.”

With the success of the VLTI and VINCI, European astronomers were able to tune the VLTI to the highest possible performance and kick-start the world’s most powerful optical/infrared interferometric facility, beginning a new era of exciting science and technology.

The VLT Interferometer (VLTI) combines the light captured by two or three 8.2-metre Unit Telescopes (UTs) or two or three 1.8-metre Auxiliary Telescopes (ATs), allowing astronomers to see details up to 25 times finer than with the individual telescopes. While the VLT´s ATs are entirely dedicated to interferometry, the VLT´s UTs can work either individually or together in groups of two or three. Credit: ESO.

“It took us two and a half nights to see the fringes with the siderostats and people were starting to be worried that something could be wrong,” says Philippe. “Most of us were working day and night, doing technical work during the day and observing at night, so we were very tired. There were two rooms in the VLTI building where we were working, and I remember people napping on couches or chairs in one of the two rooms. And then at a certain point we got woken up because nice fringes had just popped up, it was amazing! This moment for me is present as if it was last night!”

But the fact that it took two and a half days for this process, much longer than the team expected, meant that something was off. But what could that be?

It turned out that the problem was the length of the paths that the two light beams had to cover between the star and the instruments. “You have to make sure that at any time the lengths of the two paths are exactly the same, or you won’t be able to successfully combine the beams,” explains Philippe. “This is very challenging, because stars move in the sky as the Earth rotates, so we have to constantly adjust the positioning of the instruments within a fraction of the wavelength of the collected light. Since the VLTI observes infrared radiation, this means we have to be accurate down to a thousandth of a millimetre.”

How did Philippe and the rest of the team understand where the error in the length of the paths came from? “We suspected that the map of the Paranal grid, which defined where we had installed the siderostats, was not correct,” he says, “so we actually measured the alignment of the grid by looking at stars at night and determining the position of the North with a theodolite… just like sailors on boats with sextants centuries ago! We found the orientation of the grid to be a fraction of a degree off, meaning we were around 200 millimetres off with our instrumentation… an offset which made us wait two and a half nights to see the first fringes!”

After the first fringes with the siderostats, the team focused on getting ready for the fringes with the UTs. And indeed, six months later the VLTI finally opened its eyes. “I remember someone said that ‘I will never look at stars anymore as points’ as now we can see their shape, we can see they are disks,” concludes Philippe.

Like werewolves

“There were quite a lot of people that night, and we had a lot of fun afterwards. It’s always great when you have done something new,” remembers Markus Schöller, a staff astronomer at ESO who uses the VLTI to study binary systems of stars orbiting each other

“Since the first fringes, we have tested a lot of VLTI instruments in that room. There were empty bottles of champagne with the names of the instruments and the dates they were commissioned on them,” continues Markus.

The first instrument was VINCI, followed by MIDI in December 2002, both able to combine the light of two telescopes.

MIDI is the mid-infrared (N-band = 8 to 13 μm) instrument of the VLT interferometer. Credit: ESO.

“Just two telescopes because it’s easier, and before you run, you first want to walk,” says Markus.

MIDI was ideal for observing the dusty environment around young and evolved stars, and around black holes at the centres of galaxies. But like for VINCI, the fact that it could only combine two telescopes meant there were important limitations to what it could actually achieve.

“With two telescopes you can only figure out symmetric structures. This means that if you are observing two stars and one has a spot on its surface, you will never be able to say where it is,” Markus adds.

Some help comes from the rotation of the Earth, which changes the orientation of the interferometer relative to the object being observed. Still, combining more than two telescopes is a must to improve the performance of the interferometer.

A major step forward in this regard was AMBER, an instrument which saw first light in March 2004 and was capable of combining the light of three telescopes.

AMBER (a creative acronym for Astrometrical Multi BEam combineR) is one of the VLTI instruments. It combines the light of 3 telescopes, and disperses to analyse its spectrum. The very complex optical table is required to clean up and adjust the beams from the 3 telescopes. Credit: ESO.

Thanks to its better ability to detect very fine detail, AMBER has allowed astronomers to discover large bubbles of gas moving in the atmosphere of the red supergiant star Betelgeuse, for example.

But wait: how do astronomers decide when the UTs are used as standalone telescopes or combined with the VLTI? “With interferometers we’re looking at bright stars, so we don’t care so much about the Moon,” says Markus [1]. “So, if you go and look into every VLT observation schedule, the VLTI is always around the full Moon. That means that your whole life is driven by the Moon. We had years where every full Moon or let’s say 10 out of 12 full Moons we were doing something technical to the interferometer and I was at Paranal 90% of the time.” It’s not only werewolves who are strongly influenced by the full Moon!

The end of a journey is the beginning of another

The first fringes seen with the two UTs, which concluded the long design and test phase, marked the beginning of the journey for the VLTI.

Between 2004 and 2006, four 1.8-metre diameter Auxiliary Telescopes (ATs) were installed at Paranal and joined the VLTI. The ATs combine their light similarly to the UTs, with the first fringes with two ATs being observed in February 2005. But unlike the UTs, the ATs are not static and can be moved to 30 different positions across the platform, up to 200 metres [2], enabling the VLTI to probe cosmic objects 25 times more sharply than with the individual UTs.

And the improvements didn’t end here: a new generation of instruments was installed between 2010 and 2018, all currently operational: MATISSE, PIONIER and GRAVITY, all of them able to connect four telescopes.

ESO CNRS VLT Matisse Multi-AperTure mid-Infrared SpectroScopic Experiment.

ESO VLTI PIONIER instrument [First light October 2010]


When light from all four 8.2-metre Unit Telescopes of ESO’s Very Large Telescope (VLT) in Cerro Paranal on 17 March 2011 was successfully combined for the first time (ann11021), ESO Photo Ambassador Gerhard Hüdepohl was there to capture the moment.

Having all four of the Unit Telescopes (UTs) working as one telescope observing the same object was a major step in the development of the VLT. While mostly used for individual observations, the UTs were always designed to be able to operate together as part of the VLT Interferometer (VLTI).

All the UTs are pointed in the same direction, at the same object, although this isn’t obvious because of the wide-angle lens used to take the photo. The light collected by each of the telescopes was then combined using an instrument called PIONIER [3]. When combined, the UTs can potentially provide an image sharpness that equals that of a telescope with a diameter of up to 130 metres.

Two of the four 1.8-metre Auxiliary Telescopes, which are also part of the VLTI, can be seen in the picture together with the UTs. While the larger telescopes are fixed, these smaller instruments, in round enclosures, can be relocated to 30 different stations. With the ATs as part of the VLTI, astronomers can capture details up to 25 times finer than with a single UT.

Gerhard Hüdepohl has lived in Chile since 1997. Aside from taking stunning photos in the Atacama Desert, he works as an electronics engineer at the VLT.


Over its two decades of activity, the VLTI has helped us deepen our understanding of many research fields in astronomy by collecting a stunning series of groundbreaking results, such as the ​​first direct observation of an exoplanet using optical interferometry, the best ever image of a star’s surface and atmosphere, the sharpest view of a dusty disc around an aging star, or testing general relativity close to the black hole at the centre of our galaxy.

Yet the VLTI is not tired of collecting scientific results, and it continues to unveil the secrets of the cosmos and help humanity understand its place in it. “I personally think the future of the VLTI is super bright,” concludes Markus. “Twenty years ago, I would have not even dreamed what we’ve achieved so far, and what people are thinking to do in the next 10 or 15 years, I would have thought it was pure science fiction.”

For what you have done and will do, thank you, VLTI!

And happy birthday!


[1] Observations of faint objects with the standalone UTs require a dark sky with little or no moonlight. Wavelength also plays a role: at optical wavelengths, moonlight is scattered around and makes the sky brighter. Infrared light isn’t scattered as much; moreover, at these wavelengths the brightness of the sky is due to the thermal emission of the atmosphere. Because the VLTI observes bright sources at infrared wavelengths, observations using the UTs are scheduled around full Moon. The rest of the time, the VLTI uses the smaller Auxiliary Telescopes.

[2] Currently the length of the delay lines only allows to use baselines up to 140 m long, but there are ongoing works to extend this up to 200 m.

[3] PIONIER, developed at LAOG/IPAG in Grenoble, France, is a visiting instrument at the Paranal Observatory. PIONIER is funded by Université Joseph Fourier, IPAG, INSU-CNRS (ASHRA-PNPS-PNP) ANR 2G-VLTI ANR Exozodi. IPAG is part of the Grenoble Observatory for Sciences of the Universe [Observatoires des sciences de l’Univers de Grenoble](FR).

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European Southern Observatory [Observatoire européen austral][Europäische Südsternwarte] (EU) is the foremost intergovernmental astronomy organisation 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. ESO carries out an ambitious programme 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 organising 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 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-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

European Southern Observatory(EU) La Silla HELIOS (HARPS Experiment for Light Integrated Over the Sun).

ESO 3.6m telescope & HARPS atCerro LaSilla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres.

MPG Institute for Astronomy [Max-Planck-Institut für Astronomie](DE) 2.2 meter telescope at/European Southern Observatory(EU) Cerro La Silla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres.

European Southern Observatory(EU)La Silla Observatory 600 km north of Santiago de Chile at an altitude of 2400 metres.

European Southern Observatory(EU) , Very Large Telescope at Cerro Paranal in the Atacama Desert •ANTU (UT1; The Sun ) •KUEYEN (UT2; The Moon ) •MELIPAL (UT3; The Southern Cross ), and •YEPUN (UT4; Venus – as evening star). Elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo.

European Southern Observatory(EU)VLTI Interferometer image, Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level, •ANTU (UT1; The Sun ),
•KUEYEN (UT2; The Moon ),
•MELIPAL (UT3; The Southern Cross ), and
•YEPUN (UT4; Venus – as evening.

ESO Very Large Telescope 4 lasers on Yepun (CL)

Glistening against the awesome backdrop of the night sky above ESO’s Paranal Observatory, four laser beams project out into the darkness from Unit Telescope 4 UT4 of the VLT, a major asset of the Adaptive Optics system.

ESO/NTT NTT at Cerro La Silla , Chile, at an altitude of 2400 metres.

Part of ESO’s Paranal Observatory, the VLT Survey Telescope (VISTA) observes the brilliantly clear skies above the Atacama Desert of Chile. It is the largest survey telescope in the world in visible light, with an elevation of 2,635 metres (8,645 ft) above sea level.

European Southern Observatory/National Radio Astronomy Observatory(US)/National Astronomical Observatory of Japan(JP) ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres.

European Southern Observatory(EU) ELT 39 meter telescope to be on top of Cerro Armazones in the Atacama Desert of northern Chile. located at the summit of the mountain at an altitude of 3,060 metres (10,040 ft).

European Southern Observatory(EU)/MPG Institute for Radio Astronomy [MPG Institut für Radioastronomie](DE) ESO’s Atacama Pathfinder Experiment(CL) high on the Chajnantor plateau in Chile’s Atacama region, at an altitude of over 4,800 m (15,700 ft).

Leiden MASCARA instrument cabinet at Cerro La Silla, located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft).

ESO Next Generation Transit Survey telescopes, an array of twelve robotic 20-centimetre telescopes at Cerro Paranal,(CL) 2,635 metres (8,645 ft) above sea level.

ESO Speculoos telescopes four 1 meter robotic telescopes at ESO Paranal Observatory 2635 metres 8645 ft above sea level.

TAROT telescope at Cerro LaSilla, 2,635 metres (8,645 ft) above sea level.

European Southern Observatory(EU) ExTrA telescopes at erro LaSilla at an altitude of 2400 metres.

A novel gamma ray telescope under construction on Mount Hopkins, Arizona. A large project known as the Čerenkov Telescope Array composed of hundreds of similar telescopes to be situated in the Canary Islands and Chile at, ESO Cerro Paranal site The telescope on Mount Hopkins will be fitted with a prototype high-speed camera, assembled at the. University of Wisconsin–Madison and capable of taking pictures at a billion frames per second. Credit: Vladimir Vassiliev.

European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU), The new Test-Bed Telescope 2is housed inside the shiny white dome shown in this picture, at ESO’s LaSilla Facility in Chile. The telescope has now started operations and will assist its northern-hemisphere twin in protecting us from potentially hazardous, near-Earth objects.The domes of ESO’s 0.5 m and the Danish 0.5 m telescopes are visible in the background of this image.Part of the world-wide effort to scan and identify near-Earth objects, the European Space Agency’s Test-Bed Telescope 2 (TBT2), a technology demonstrator hosted at ESO’s La Silla Observatory in Chile, has now started operating. Working alongside its northern-hemisphere partner telescope, TBT2 will keep a close eye on the sky for asteroids that could pose a risk to Earth, testing hardware and software for a future telescope network.

European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU) The open dome of The black telescope structure of the‘s Test-Bed Telescope 2 peers out of its open dome in front of the rolling desert landscape. The telescope is located at ESO’s La Silla Observatory, which sits at a 2400 metre altitude in the Chilean Atacama desert.a desert.