|
Adaptive optics is a technology to improve the performance of reflecting telescopes by reducing the effects of atmospheric distortion, or twinkling. Adaptive optics work by measuring the distortion and rapidly reshaping telescopic mirrors. While the technique was theoretically understood for some time, it was only advances in computer technology during the 1990s that finally made the technique practical. Adaptive optics should not be confused with active optics, which works on a longer timescale to correct the mirror geometry itself.
When light from a star or another astronomical object enters the Earth's atmosphere, the different temperature layers and different wind speeds distort and move the image in various ways (see astronomical seeing for a full discussion). The net result is that a large telescope (8m+) (like the VLT or Keck), while theoretically capable of milli-arcsecond resolution, is limited to what the atmosphere permits, which can easily be a factor of 50 or 100 times worse.
An adaptive optics system tries to correct these distortions, using a wavefront sensor, a deformable mirror, and a computer that receives input from the detector and calculates the optimal deformation of the mirror, and then re-shapes the surface of the mirror accordingly.
This is possible only because the theoretical, perfect image is known in advance: in the case of a point-like star, the image is a circle with certain characteristics dictated by the telescope's aperture, known as a diffraction pattern.
The necessity of a reference star means that an adaptive optics system cannot work everywhere on the sky, but only where 'guide stars' of sufficient luminosity (for current systems, about magnitude 11-12) can be found very near to the object of the observation.
An alternative is the use of a laser beam to generate a target in the atmosphere, to act as a guide star - (referred to as a laser guide star).
Other approaches that can yield resolving power exceeding the limits of atmospheric seeing include lucky imaging and space telescope such as NASA's Hubble Space Telescope.
Adaptive optics technology has also been used recently to image individual cone photoreceptors in the living, human eye and is also expected to play a military role by allowing ground-based and airborne laser weapons to reach and destroy targets at a distance including satellites in orbit.
See also
active optics
External links
Category:Optics
Category:Astronomy
|