Telescope

In 1991, the United States government declassified adaptive optics systems (systems that remove atmospheric effects), which had been developed under the Strategic Defense Initiative for ensuring that a laser beam could The Clark telescope at Lowell Observatory in Arizona. Photo Researchers, Inc. Reproduced by permission. penetrate the atmosphere without significant distortion. The principle behind adaptive optical telescope systems is illustrated in Figure 3..

A laser beam is transmitted from the telescope into a layer of mesospheric sodium at 56-62 mi (90-100 km) altitude. The laser beam is resonantly backscattered from the volume of excited sodium atoms and acts as a guide-star whose position and shape are well defined except for the atmospheric distortion. The light from the guide-star is collected by the telescope and a wavefront sensor determines the distortion caused by the atmosphere. This information is then fed back to a deformable mirror, or an array of many small mirrors, which compensates for the distortion. As a result, stars that are located close to the guide-star come into a focus, which is many times better than can be achieved without compensation. Telescopes have operated at the theoretical resolution limit for infrared wavelengths and have shown an improvement in the visible region of more than ten times. Atmospheric distortions are constantly changing, so the deformable The Anglo-Australian telescope (AAT) in New South Wales, Australia. The principal mirror of this optical telescope has a diameter of 12.8 ft (3.9 m) and lies at the bottom of the white frame. During the day it is protected by an array of steel petals that are seen here in open position. The larger horseshoe shaped frame is part of the supporting structure. AAT was one of the first electronically controlled telescopes. Royal Observatory, Edinburgh/National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. mirror has to be updated every five milliseconds, which is easily achieved with modern computer technology.