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'Star Wars' generates sharper stellar images.

They were conducting research on laser weapons, not trying to take the twinkle out of starlight. But the "Star Wars" studies that an Air Force laboratory and a university research team began a decade ago -- and kept secret until last week -- have yielded a laser system that corrects for the atmosphere's blurring of celestial objects. This technology promises to dramatically sharpen the images produced by some ground-based optical telescopes, potentially putting them on a par with costlier instruments that orbit above the interfering atmosphere.

Already tested on small telescopes, the laser system could revolutionize infrared and visible-light astronomy from Earth, its developers reported last week in Seattle at a meeting of the American Astronomical Society.

Ground-based astronomy is handicapped by distortions created when starlight travels through the atmosphere -- a turbulent hodgepodge of hot and cold air masses that constantly alters its refractive index. The rapidly changing index deforms the wavefronts of incoming starlight so that the radiation appears to twinkle and come from a fuzzy blob instead of a celestial point.

Astronomers in Europe and Canada have made some headway in solving the problem with adaptive optics. This involves measuring ,the atmospheric distortion of light coming from a bright, easy-to-measure reference star near the celestial object of interest, then electrically deforming a flexible telescope mirror to optically compensate for the distortion. However, this technique has one key limitation: Most heavenly bodies don't reside near a bright reference star.

Physicists now report they have overcome this limitation by mimicking the effects of a reference star with a powerful laser. The researchers shoot an intense laser beam some 90 kilometers into the atmosphere and measure its resulting distortion as gas particles reflect the light back to Earth.

Although French scientists first publicly described such a theoretical laser-guide system in 1985, Robert C. Fugate says he and his colleagues had secretly conducted the first assays of atmospheric distortions in 1983. Fugate, of the Kirtland Air Force Base in Albuquerque, N.M., reports performing these and later imaging experiments with a 15-meter telescope at Kirtland.

Charles A. Primmerman and his group at the Massachusetts Institute of Technology's Lincoln Laboratory in Lexington made the first corrections for atmospheric turbulence with the aid of a laser beam in August 1988, working with a 60-centimeter telescope in Hawaii. The team initially used a laser at the White Sands Missile Range near Las Cruces, N.M., to assess the relatively low-altitude concentrations of nitrogen and oxygen. In 1984, they extended their distortion measurements to the sodium layer, about 90 kilometers above Earth.

Primmerman notes that for best results astronomers should use both laser light and a true reference star, since lasers cannot compensate for another atmosphere-induced phenomenon called wandering -- the apparent movement of an image. He adds that several artificial stars -- i.e., laser beams -- may be needed to eliminate distortion in flexible-mirror telescopes larger than 10 meters.

Results presented at last week's meeting by Laird A. Thompson of the University of Illinois in Urbana suggest that a laser-based adaptive optics system might give a fairly large telescope -- such as the 4-meter telescope on Kitt Peak in Arizona -- the viewing quality previously restricted to instruments orbiting above the atmosphere. He calculates taht a 4-meter telescope with the laser system could distinguish celestial features about 0.04 arc-seconds apart -- roughly twice the resolution expected from the Hubbie Space Telescope once its optical flaws are corrected.
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Title Annotation:Strategic Defense Initiative research yields a laser system that corrects atmospheric blurring of celestial objects
Author:Cowen, Ron
Publication:Science News
Date:Jun 8, 1991
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