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The eye that looks at galaxies far, far away

12 February 2016

Take a virtual tour of ESO’s breathtaking installation.

Night is falling over Cerro Paranal, a 2600 m peak within the mountain range running along Chile’s Pacific coastline. As our eyes gradually become accustomed to total obscurity and we start to catch a glimpse of the profile of the domes on top of the Cerro, we are overwhelmed by the breathtaking view of the best starry sky we have ever seen. The centre of the Milky Way is hanging over our heads, together with the two Magellanic Clouds and the four stars of the Southern Cross. The galactic centre is so star-dense that it looks rather like a 3D object suspended in the sky.

Not a single artificial light source is polluting the site, which is literally in the middle of nowhere, because the closest inhabited area is about 130 km away. The air in the austral winter in the Atacama desert is cold, but there is almost no wind, and no noise can be heard as I walk in the shadow of four gigantic (30 m tall) metal domes housing the four 8.2 m-diameter fixed unit telescopes (UTs) and four 1.8 m-diameter movable auxiliary telescopes (ATs), that make up the Very Large Telescope (VLT). Yet dozens of astronomers are working not far away, in a building right below the platform on top of the Cerro, overlooking the almost permanent cloud blanket over the Pacific Ocean.

As we enter the control room, I immediately feel a sense of déjà vu: a dozen busy and mostly young astronomers are drinking coffee, eating crisps and talking in at least three different languages, grouped around five islands of computer terminals.

Welcome to the nerve centre of the most complex and advanced optical telescope in the world. From here, all of the instrumentation is remotely controlled through some 100 computers connected to the telescopes by bunches of optical fibres. Four islands are devoted to the operation of all of the components of the VLT telescopes, from their domes to the mirrors and the imaging detectors, and the fifth is entirely devoted to the controls of interferometry.

 

Highly specialised ESO astronomers take their night shifts in this room 300 nights per year, on average. Most observations are done in service mode (60–70% of the total time), with ESO staff doing observations for other astronomers within international projects that have gone through an evaluation process and have been approved. The service mode guarantees full flexibility to reschedule observations and match them with the most suitable atmospheric conditions. The rest of the time is “visitor mode”, with the astronomer in charge of the project leading the observations, which is particularly useful whenever any real-time decision is needed.

The shift leader tonight is an Italian from Padova. He swaps from one screen to the next, trying to ignore the television crew’s microphones and cameras, while giving verbal instructions to a young Australian student. He is activating one of the VLT’s adaptive-optics systems, hundreds of small pistons positioned under the mirrors to change their curvature up to thousands of times per second, to counteract any distortion caused by atmospheric turbulence. “Thanks to adaptive optics, the images obtained with the VLT are as sharp as if we were in space,” he explains briefly, before leaning back on one of the terminals.

Complex machinery

Adaptive optics is not the only astronomers’ dream come true at the VLT. The VLT’s four 8.2 m-diameter mirrors are the largest single-piece light-collecting surface in the world, and the best application of active optics – the trick ESO scientists use to correct for gravitationally induced deformations as the telescope changes its orientation and so maintain the optics of the vast surface. The telescope mirrors are controlled by an active support system powered by more than 250 computers, working in parallel and positioned locally in each structure, to apply the necessary force to the mirrors to maintain their alignment with one another. The correcting forces have a precision of 5 g and keep the mirror in the ideal position, changing it every 3 minutes with 10 nm precision. The forces are applied on the basis of the analysis of the image of a real star, taken during the observations, so that the telescope is self-adjusting. The weight of the whole structure is incredibly low for its size. The 8.2 m-diameter reflecting surface is only 17 cm thick, and the whole mirror weighs 22 tonnes; its supporting cell weighs only 10 tonnes. Another technological marvel is the secondary mirror, a single-piece lightweight hyperbolic mirror that can move in all directions along five degrees of freedom. With its 1.2 m diameter, it is the second largest object entirely made in beryllium, after the Space Shuttle doors.

But the secret of the VLT’s uniqueness lies in a tunnel under the platform. Optical interferometry is the winning idea that enables the VLT to achieve yet unsurpassed ultra-high image resolution, by combining the light collected by the main 8.2 m UTs and the 1.8 m ATs. The physics principle behind the idea stems from Young’s 19th century two-slit experiment, and was first applied to radio astronomy, where wavelengths are long. But in the wavelength domains of visible and infrared light, interferometry becomes a much greater challenge. It is interesting to note that the idea of using optical interferometry became a real option for the VLT at the ESO conference held at CERN in 1977 (cf Claus Madsen The Jewel on the Mountain Top Wiley-VCH).

With special permission from the director and taking advantage of a technical stop to install a new instrument, we are able to visit the interferometry instrumentation room and tunnel under the platform – a privilege granted to few. The final instrument that collects and analyses all of the light coming from the VLT telescopes, after more than 25 different reflections, is kept like a jewel in a glass box in the instrumentation room. Nobody can normally get this close to it, because even the turbulence generated by a human presence can disturb its high-precision work. Following the path of the light, we enter the interferometry tunnel. The dominant blue paint of the metal rails and the size of the tunnel trigger once again an inevitable sense of déjà vu. Three horizontal telescopes travel seamlessly on two sets of four 60 m-long rails – the “delay lines” where the different arrival times of photons on each of the telescopes is compensated for with ultra precision. These jewels of technology move continuously along the rails without electric contact, thanks to linear engines with coils interacting directly with the magnets in the engine; no cable is connected to the telescopes on the rails because the signals are transmitted by laser, and electricity is conveyed by the rails themselves to enable precision and smooth movement. The system is so precise that it can detect and automatically adapt to earthquakes, and measure the vibrations provoked in the mountain by the waves of the Pacific Ocean 12 km away. Nowhere else has interferometry reached such complexity and been pushed so far.

Delivering science at a high rate

The resolution obtained by the Very Large Telescope Interferometer (VLTI – the name given to the telescopes when they function in this mode) is equivalent to the resolution of a 100 m-diameter mirror. Moreover, the Auxiliary Telescopes are mounted on tracks, and can move over the entire telescope platform, enabling the VLTI to obtain an even better final resolution. The combined images of the 4+4 telescopes allow the same light collection capacity as a much larger individual mirror, therefore making the VLT the largest optical instrument in the world.

Up to 15% of refereed science papers based on ESO data are authored by researchers not involved in the original data generation

Another revolution introduced by the VLT has to do with e-science. The amount of data generated by the new high-capacity VLT science instruments drove the development of end-to-end models in astronomy, introducing electronic proposal submission and service observing with processed and raw science and engineering data fed back to everyone involved. The expansion of the data links in Latin America enabled the use of high-speed internet connections spanning continents, and ESO has been able to link its observatories to the data grid. “ESO practises an open-access policy (with regulated, but limited propriety rights for science proposers) and holds public-survey data as well. Indeed, it functions as a virtual observatory on its own,” says Claus Madsen, senior counsellor for international relationships at ESO. Currently, up to 15% of refereed science papers based on ESO data are authored by researchers not involved in the original data generation (e.g. as proposers), and an additional 10% of the papers are partly based on archival data. Thanks also to this open-access policy, the VLT has become the most productive ground-based facility for astronomy operating at visible wavelengths, with only the Hubble Space Telescope generating more scientific papers.

Watch the video at https://cds.cern.ch/record/2128425.

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