Adaptive optics

Adaptive optics is the future of large telescopes. It is a new and exciting technique that is still developing quickly and has already demonstrated dramatic improvements of image quality on several large night-time telescopes around the world, e.g. the Canada-France Hawaii Telescope (CFHT), Gemini North, Keck and ESO VLT.

The principle of adaptive optics is simple but its implementation difficult. To build an adaptive optics system you need a deformable mirror and a wavefront sensor.

The wavefront sensor can be of different types. A common type is called the Schack-Hartmann wavefront sensor which looks at a single star through many small parts of the telescope aperture and measures the position of the star as seen through each part. When the atmosphere disturbs the image, it causes these images to move differently depending on their position in the aperture.

The positions are measured and translated to commands to the deformable mirror, so that it takes the shape that compensates for the distortions. The problem is that the atmosphere changes quickly so this has to be done very accurately and at a very high frequency. Adaptive optics systems have to correct the adaptive mirror at least several hundred and preferably more than 1000 times per second.

Measuring the position of a star can be done easily but requires the star to be bright enough that there is sufficient light to do this quickly enough. For night time astronomy a problem is that there may often not be a sufficiently bright star near the object that the astronomer wants to observe. Several projects are therefore in progress that use a laser to create an artificial star very high up in the Earth's atmosphere.

For solar telescopes, there are no stars or star-like objects that can serve as reference for a wavefront sensor. However, there is solar fine structure everywhere on the Sun's surface. The position of such structure can also be measured but requires much more complicated calculations than for a night-time adaptive optics system. There are therefore very few solar adaptive optics systems in operation. The first truly functioning solar adaptive optics system was built at the Sacramento Peak Solar Observatory for the R. B. Dunn telescope. This was followed a few months later by the system built for the previous Swedish 50 cm solar telescope in La Palma. Without such a system, it would not be meaningful to build the new 1-meter solar telescope.

The Canada-France Hawaii Telescope (CFHT)

Gemini North



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