From ESOblog (EU): “Blow out your twenty candles, VLTI!”

From ESOblog (EU)


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

29 October 2021
On the Ground

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

Giulio Mazzolo
It is the evening of the 29th of October 2001 on the top of Cerro Paranal in the Chilean Atacama Desert, twenty years ago today, and there’s more than the usual dose of excitement in the air. A group of astronomers and engineers are busily working on the platform of ESO’s observatory, getting ready for a night that will forever change the way we probe the cosmos from Paranal. After years of preparatory work, the Very Large Telescope Interferometer (VLTI) is about to open its eyes, sparking the beginning of two decades of stunning scientific discoveries. In this blog post, the first of a two-part series, we celebrate the twentieth anniversary of that historical night with the memories of some of the scientists who experienced it in person.

Original aerial view of the observing platform on the top of Paranal mountain (from late 1999), with the four enclosures for the 8.2-m Unit Telescopes (UTs) and various installations for the VLT Interferometer (VLTI). Three 1.8-m VLTI Auxiliary Telescopes (ATs) 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 centre of the platform. Credit:ESO.

“In the beginning there were the four Unit Telescopes (UTs), perched on Cerro Paranal and making up ESO’s Very Large Telescope (VLT). The idea had always been to combine the light collected by the UTs with a technique called interferometry. Why? Because this creates a “virtual”, giant telescope — the VLTI — which is as large as the distance between the linked UTs, located up to 130 metres apart on the Paranal platform. And since a telescope’s sharpness increases with its diameter, the VLTI would allow to discern much finer details than the individual UTs, whose main mirrors are 8.2 metres large.

Yet big rewards only come with big risks. Linking the UTs with interferometry was an enormous technological challenge. For example, the light collected by the UTs had to be sent through a complex system of mirrors in underground tunnels positioned with a precision of around one thousandth of a millimetre!

A feat this scale required years of careful planning and testing. One major test of the feasibility of the VLTI was passed on March 17, 2001, when astronomers and engineers managed to successfully combine the light from the star Sirius collected by two small test telescopes (the “siderostats”) and see the “first fringes” — the patterns resulting from the interference of both lightwaves.

This proved that the VLTI concept could work, and the VLTI staff spent the following months getting ready for the moment of truth, set for the 29th of October 2001: combining the light of two UTs.

MacGyver at Paranal

“Back then I was at Paranal as I was responsible for the mirrors M9, M10 and M11 of the UTs,” remembers Françoise Delplancke-Ströbele, now Executive Officer for the Directorate of Engineering at ESO, referring to the mirrors under the UTs that send the light towards the VLTI tunnels. “M9 was a big mirror, very difficult to make, and the company who had to produce it was late, so the mirror was coming the day after the first fringes.”

But everything was ready to point the UTs to Achernar, the star chosen for the first fringes, so Françoise had to quickly find a solution. Fortunately, for that very first observation, not all of M9 and its special features were needed, so she found two smaller and simpler mirrors (one for each UT to be used for the observation) and went to the Paranal workshop. “I asked if they could make me two pieces of wood with a hole in them to sustain the mirrors, and then we used them in the telescope instead of the original mirror. It was a very bad quality wood, but this sort of ‘MacGyver bricolage’ worked well enough!”

Schematic lay-out of the VLT Interferometer. The light from a distant celestial objects enters two of the VLT telescopes and is reflected by the various mirrors into the Interferometric Tunnel, below the observing platform on the top of Paranal. Two Delay Lines with moveable carriages continuously adjust the length of the paths so that the two beams interfere constructively and produce fringes at the interferometric focus in the laboratory. Credit: D. Catricheo/ESO.

A view along one half of the Interferometric Tunnel. Credit: H.H.Heyer/ESO.

With the MacGyver solution in place, everything was ready for the light from the two UTs to be combined. Except, the sky conditions that night weren’t great. Thinking the observations couldn’t take place, Françoise went to bed while her colleagues operated the interferometer. “And then, in the middle of the night, I got a phone call: ‘we’ve got the first fringes, come!’, so I got out of bed and went to celebrate!” In case you are wondering, when they installed the official mirrors the following day, the system still worked. Phew!

That first observation of the VLTI enabled the calculation of Achernar’s size. This was the first of the many scientific results made possible by the VLTI’s stunning sharpness, which could resolve details about 10 m wide at the distance of the Moon.

The sharpness of the VLTI becomes even more impressive when thinking about the whole concept behind it. Basically, two lightwaves emitted at the same moment from a star wander undisturbed through the galaxy until they reach the Earth, and those last few kilometers scramble everything! As lightwaves go through the atmosphere, air turbulence changes their directions and time of arrival at the telescopes. They then have to pass through 200 metres of air in the underground tunnels of the telescope and undergo 30 reflections on the mirrors, before meeting again at the same moment and at the same place within a fraction of a millionth of a meter, depending on the VLTI instruments.

Francoise Delplancke adjusting the PRIMA instrument, which is partially accessible from the top only. Credit:

To achieve such precision, the VLTI must account for a multitude of factors, including any tiny source of vibrations, such as the wind… or earthquakes! This is very challenging in a country as seismic as Chile. “Once I was at the console of the VLTI making some observations and we started to feel an earthquake, which was magnitude three or four,” remembers Françoise. “After the earthquake, we retook the measurement, as the vibration of the telescopes caused by the earthquake could have disturbed the observation… and we got exactly the same result, as the VLTI is built in a way that measures the vibrations and cancels their effect.”

Dancing Cueca

“I was in charge of VINCI, so the night of the first fringes I had to be at Paranal,” says Pierre Kervella, Astronomer at Paris Observatory (LESIA), and former ESO Staff and Fellow in Chile and ESO student in Garching, referring to the VINCI test instrument installed on ESO’s VLTI for the first fringes and subsequent test phase.

“VINCI was used to validate the infrastructure of the VLTI, which is quite complex, so it was imagined to be simple, as we don’t need another complicated instrument to validate a complicated installation. Yet it had to be reliable and very precise, more precise than anything existing at that time,” explains Pierre, whose doctoral studies were mainly dedicated to building VINCI. “When I started my PhD, the instrument was a manual sketch on a sheet, and at the end of the PhD it had reached first fringes and scientific results.”

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. Credit: ESO.

A few months earlier the interferometer at Keck Observatory in the US had successfully combined the light of its two 10-m telescopes, and the team was motivated by this exciting competition. The interferometer at Keck Observatory is no longer in service.

W.M. Keck Observatory two ten meter telescopes operated by California Institute of Technology(US) and The University of California(US), at Mauna Kea Observatory, Hawaii USA, altitude 4,207 m (13,802 ft). Credit: Caltech.

The night when VINCI enabled the VLTI to see the first fringes with two UTs is still a vivid memory for Pierre. “That night was not supposed to be for scientific observations, only for testing. But if you are an astronomer and have the biggest instrument in the world and the whole sky for yourself…” remembers Pierre, who adds: “I could choose whatever star and have a look at it, it was an incredible feeling. All the famous objects in the southern sky, such as Eta Carinae or Epsilon Eridani, were at hand, it was the first time we could look at them in such huge detail!”

The following morning Pierre went to sleep, exhausted after weeks of working up to 16 hours per day. “Despite the tiredness, there was a very nice spirit in the team, because we were doing something uniquely challenging,” says Pierre, who did not have long to rest, as the date of his PhD defense was just behind the corner.

“Two weeks after the first fringes I defended my PhD at the Paris Observatory. It was perfect timing, as I could show the first fringes during the defence, with my parents, friends and everyone present, it was nice!” continues Pierre “And of course I remember the big party we had in Chile to celebrate the fringes, which is when I learnt how to dance Cueca, Chile’s national dance!”

To be continued…

Françoise and Pierre are only two of the many key actors that made the VLTI a reality. In the second article in this series we will hear more stories about the VLTI first light; in particular, we will learn what the VLTI and werewolves have in common, and how a centuries-old sailing technique was key to correct a puzzling problem. Stay tuned!

See the full article here .


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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.