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Adaptive optics is a technology to improve the performance of astronomical telescopes by reducing the effects of atmospheric distortion, or twinkling. Adaptive optics works by measuring the distortion and rapidly compensating for it either using deformable mirrors or material with variable refractive properties. 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 primary mirror geometry itself.
When light from a star or another astronomical object enters the Earth's atmosphere, turbulence introduced (for example, by different temperature layers and different wind speeds interacting) distort and move the image in various ways (see astronomical seeing for a full discussion). Images produced by any telescope larger than a few centimeters are blurred by these distortions. For example, a 2.5m telescope is reduced in resolution by a factor of between 7 and 20; in the case of very large telescopes (8m+) (like the VLT or Keck), which are theoretically capable of milli-arcsecond resolution, the loss can be a factor of 50-100.
An adaptive optics system tries to correct these distortions, using a wavefront sensor which takes some of the astronomical light, a deformable mirror that lies in the optical path, and a computer that receives input from the detector. The wavefront sensor measures the distortions the atmosphere has introduced on the timescale of a few milliseconds; the computer calculates the optimal mirror shape to correct the distortions and the surface of the deformable mirror is reshaped 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 (technically an Airy Disc ) with certain characteristics dictated by the telescope's aperture, known as a diffraction pattern. The system can then correct incoming light from that star so it appears as a point source. Because a science target is often too faint to be used as a reference star a nearby brighter guide star is used instead. The light from the science target has passed through approximately the same atmospheric turbulence as the reference star's light and so its image is also corrected, although generally to a lower accuracy.
The necessity of a reference star means that an adaptive optics system cannot work everywhere on the sky, but only where guide star of sufficient luminosity (for current systems, about magnitude 11-12) can be found very near to the object of the observation. This severely limits the application of the technique for astronomical observations.
An alternative is the use of a laser beam to generate a target (a Laser Guide Star , LGS) in the atmosphere. The LGS can then be used as a wavefront reference in the same way as a natural guide star - except that (much fainter) natural reference stars are still required for image position (tip/tilt) information.
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.
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