What is the Keck telescope used for?

The technology of the giant: See extremely sharply in the infrared with the ELT

There is a lot of new technology in the new giant telescope "Extreme Large Telescope", which draws on the tried and tested. But this size poses completely new challenges for technicians and developers in order to do precise work.

The astronomers have clear requirements for the new star among telescopes, explains Mark McCaughrean of Exeter University: “The 42-meter telescope has to be big enough to capture even very faint objects. It also has to see perfectly sharp. "

Adaptive optics for the ELT

To do this, the astronomers switch off the air turbulence with the adaptive optics. An extremely flexible and two and a half meter large mirror is deformed by more than 5000 stamps up to 1000 times per second in such a way that it precisely compensates for the flickering of the atmosphere. "Then our telescope sees as sharply as if it were in space," explains chief scientist Roberto Gilmozzi. But he also admits: "This is the greatest technological challenge that we have to master."

Laser mimics artificial star

There are already adaptive optics on the telescopes in Chile - but they work with 150 stamps. Because so far the adaptive optics have only been used in the instruments attached to the telescope. With the new giant telescope, the adaptive optics will be directly part of the telescope and will always be in operation. To do this, the quality of this technology, i.e. the speed and precision of the image analysis as well as the deformation of the correction mirror in real time, must at least double, and in some cases even quadruple.

A mirror made of 906 parts

The astronomers and engineers in this project rely on enormous innovations in optics, electronics and mechanics. Originally, ESO even planned a 100 meter telescope called "OWL" (Overwhelmingly Large Telescope). Since this project did not seem feasible within ten years, a smaller version of "only" 42 meters in diameter is now favored. Roberto Gilmozzi and his team are building a gigantic puzzle: “The mirror consists of 906 parts - each one is 1.45 meters in diameter. The parts are hexagonal and form the mirror almost like the eye of an insect. ”Today the largest individual mirrors are a good eight meters high - but a 42-meter mirror is technically impossible as a single piece, even the 10-meter mirror of the Keck Telescopes in Hawaii are made up of many small segments. Servomotors align the 906 mirror parts perfectly - technically extremely demanding, because this has to be done with an accuracy of a few hundred thousandths of a millimeter.

Competitive project TMT

Roberto Gilmozzi also has an even more weighty problem: “The telescope weighs a good 5000 tons. It is extremely time-consuming to align such an instrument with the celestial objects with the highest precision. ”Some radio telescopes even weigh more than 7,000 tons. But with the long wavelengths used by radio astronomy, the precision of the control of the telescope only depends on a few millimeters. In the area of ​​visible light or near infrared, the requirements are much higher. All optical parts of the new giant telescope can be controlled in a targeted manner so that the instrument is always perfectly focused.

5000 tons blown with the wind

Because even supposedly very banal things become a big problem with this project, Gilmozzi groans: “The wind is a tricky problem for all telescopes. It makes the whole telescope swing slightly. ”In the past, astronomers would have completely protected the telescope structure, which towers almost 80 meters, from the wind in a dense dome. But then air bubbles rising from the dome blur the image. Today, wind blowing gently through the telescope is considered optimal because it ideally balances the temperature. To do this, the telescope must be protected against vibrations and, above all, against treacherous natural oscillations. The astronomers plan the entire instrument in the computer and then calculate the behavior in different weather and the changing orientation towards the sky, i.e. whether the telescope is looking at the zenith or quite low at the horizon.

“We do it like in aircraft construction - the Airbus 380 was designed entirely in the computer before it took off for the first time. Now we hope that our telescope “flies” in the end ... ”, explains Mark McCaughrean.