One extremely large telescope

The top of a 3,000 metre high mountain in Chile was blasted away on June 19 2014 to make way for the world’s largest optical and infrared telescope, the E-ELT.

E-ELT stands for European Extremely Large Telescope. This new ground-based telescope, built by the European Southern Observatory, should be operational in 2023. It will have a 39 metre main mirror and will be the largest optical/near-infrared telescope in the world. Astronomers will use it to look further into space and in more detail than ever before, enabling them to, for instance, track down Earth-like planets around other stars in the habitable zones where life could exist.

Prof. Michel Verhaegen, of the Systems and Control Group in the 3mE-faculty, watched the live-streamed explosion with great interest. He is leading a team of Delft researchers that works on the 2.4 meter wide M4 mirror, which must remove the twinkling in starlight before this light hits the main mirror.

“It was a very exciting moment”, says Verhaegen. “Not many places on earth are suitable for this kind of telescope. It took quite some time to find the right location. So, it was very important that they didn’t blow up too much of the mountain”, he laughs.

The site, in the middle of the Atacama desert, and close to the Very Large Telescope, has been chosen because the sky is cloudless most of the year.

M4 is a deformable mirror which compensates for variations in the refractive index caused by turbulence in the atmosphere, which in turn leads to blurry images. To compensate for variations in the refractive index, the deformable mirror is placed in the optical path of the telescope. When part of a wavefront from a far away celestial object is slowed down during the last small segment of its journey through our atmosphere, it will hit an elevated section of the mirror. Meanwhile, the rest will hit a valley. With this kind of compensation, all photons will arrive on the telescope’s detector simultaneously.

That, at least, is the idea. Verhaegen and his colleagues from Delft and scientists from the Dutch space research agency Sron, with whom they collaborate on this project, still need to demonstrate the technique. They will do so in about six moths, using the Nordic Optical Telscope at La Palma.

The shape of the mirror is defined by 40,000 actuators, which are adjusted every 0.3 milliseconds based on information about the incoming light which is collected by about a hundred thousand sensors. Developing a control system that can efficiently regulate this kind of set up is quite a feat.

“We can only regulate this mirror by doing parallel calculations”, says Verhaegen. This implies dividing the deformable mirror into different modules that can communicate with one another.

Verhaegen received a 2.5 million euro ERC grant (European Research Council grant) earlier this year to perform these types of parallel calculations. They are essential when huge amount of data come into play, whether you are studying fluid dynamics, traffic flows or trying to remove the twinkling in star light.

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