Mirrors help sharpen images from space. Newly tested devices may compensate for atmospheric distortion
While media attention focused on so-called ``star wars'' laser tracking during Discovery's recent mission, the National Optical Astronomy Observatory (NOAO) reported successful tests of comparable technology. They were made for an entirely peaceful purpose: to give ground-based telescopes a clearer view of the sun and stars. The shuttle experiment showed how movable mirrors can cancel atmospheric distortion of light waves. This allowed a laser beam shot from Earth to focus accurately on a target in space. The NOAO tests are part of a program to use such mirrors to cancel atmospheric blurring of telescopic images.
This would largely free astronomers from an annoying limitation that has plagued their cosmic observing ever since precision telescopes were invented.
Fascination with the ``star wars'' test overshadowed the astronomical news. This was unfortunate, for that news puts the Strategic Defensive Initiative (SDI) research program in a useful perspective. It illustrates a fact frequently emphasized by the SDI director, Lt. Gen. James A. Abrahamson, himself. The program includes many research projects -- some big, some small -- which often carry forward technology that is useful for, and being developed for, other purposes.
Both military and civilian researchers were working on adaptive mirrors, to use their technical name, before SDI was started. Astronomers have suspected that such mirrors could eliminate much of the blurring that prevent ground-based telescopes from producing a sharp image.
These suspicions were confirmed by experiments conducted May 14 and 15 at the National Solar Observatory on Sacramento Peak in New Mexico. The solar pictures obtained had a ``resolution equivalent to that which would be expected from a telescope in space,'' NOAO said in announcing the results June 20, one day before the successful shuttle laser test.
Turbulence tilts, or bends, the wave front of light moving through the air. This blurs a star image and makes it dance. It disperses and deflects a laser beam. An adaptive mirror system senses this distortion and, in effect, straightens out the wave front tilt, NOAO explains. Or it can ``predistort'' light waves in such a way that atmospheric turbulence corrects the distortion.
Such a system on a Hawaiian mountain sensed the distortion in the laser beam reflected from Discovery. It then predistorted the laser light in such a way that the beam was undistorted and well behaved after it traversed the atmosphere and entered space.
The system used in the ``Sac Peak'' tests acts differently. It has 19 separate, precisely controlled hexagonal mirror segments. As distortion in the incoming light is sensed, the mirrors are adjusted to cancel it. Robert C. Simpson of the Lockheed Palo Alto Research Laboratory, who has led the system's development, said the tests not only produced sharper images, they also showed the system has a useful field of view 10 times larger than expected. It can cover entire active regions of the sun rather than only a small part of such regions.
Adaptive mirrors will not obviate a need for space-based telescopes. Astronomers use such telescopes for observing at infrared, X-ray, and other wavelengths which do not penetrate the atmosphere well. But sharper, ground-based telescopes will greatly enhance general observing ability. Lockheed scientist Alan Title says that high-resolution, ground-based telescopes will be especially useful for in-depth studies of particular phenomena which would require too much of the time of orbiting instruments.
This system is not yet ready for working astronomers. So far, it has only been tested on very bright sources and hasn't been tried at night. Also, in tests so far, funded by the United States Air Force as well as by Lockheed, it works best under favorable seeing conditions. But Mr. Title says it can deliver images virtually as sharp as those obtained in space.
Now the team wants to build a 37-element mirror, which should form sharper images under poorer conditions than can the 19-element system.
As in this case of adaptive mirrors, many advanced technologies now under development will be attractive to SDI planners. They include a variety of special durable materials, computer programs, rocketry, and, of course, lasers. Their ``star wars'' connection may make them newsworthy. But this should not obscure the fact that their major utility may lie elsewhere.
A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.