Who Needs the Hubble?

By MICHAEL D. LEMONICK

In the first harsh light of revelation, news that the Hubble Space Telescope was flawed appeared to be an unmitigated disaster. Because the telescope's main light-focusing mirror had been precision ground to the wrong specifications, the U.S. had evidently spent $1.5 billion on an instrument that may never take the promised supersharp pictures of the heavens.

Now the situation does not look so bleak. The Hubble blunder is a serious setback, but NASA engineers have found ways to computer-enhance the telescope's images so that they are not as blurry as the first ones received. More important, in the 12 years it took to develop and launch the Hubble, the technology of ground-based telescopes has rapidly improved. In fact, techniques under development could enable earthbound telescopes to do many things the Hubble was supposed to do.

Scientists wanted a telescope in space so that the instrument would be free from temperature changes and the pull of gravity, both of which can subtly distort the shape of earthbound mirrors. They also wanted it to rise above earth's turbulent atmosphere, whose constant roiling makes the stars appear to flicker. But scientists have learned to make mirrors that can change their shapes, enabling ground-based telescopes to overcome the problems of gravity and temperature fluctuations. Soon it may be possible to compensate for the atmosphere's turbulence as well.

The best example of a shape-changing "active optics" mirror is the one in the European Southern Observatory's New Technology Telescope in La Silla, Chile. Pistons attached to the thin mirror can flex it in and out until a star is as focused as possible. The NTT has already produced some of the sharpest images ever taken from the ground. A comparable system will be used in other projects, including the giant Keck Telescope under construction atop Mauna Kea in Hawaii.

Active-optics mirrors can refocus in seconds, but the atmosphere's turbulence can make a star seem to flicker hundreds of times a second. Compensating for the flicker calls for a still experimental system called adaptive optics. Different versions of the equipment are being developed at the University of Hawaii and Johns Hopkins University, as well as in Europe.

Adaptive optics depends on taking starlight focused by a telescope's main mirrors and bouncing it off yet another mirror before studying the image. The additional mirror is made of a superflexible material -- plastic, in the Johns Hopkins device. A light sensor monitors a reference star within the telescope's field of view and looks for the shimmering caused by currents in the atmosphere. When the sensor detects disturbances, it sends signals to electrodes flanking the plastic mirror. The electrodes create electric fields that make the plastic bulge or dip, canceling out the flicker. Both the Hawaii and Johns Hopkins teams expect to test their mirrors early in 1991. If they work, the adaptive-optics systems could be used on virtually any existing telescope.

For all its problems, the Hubble will not be a total waste of money. As it is, the telescope is unsurpassed in the detection of ultraviolet light. And if NASA is able to send up astronauts to fix the Hubble's flawed mirror -- a mission tentatively planned for 1993 -- the telescope should be able to take somewhat sharper pictures than adaptive-optics systems can. But whether or not the repair effort succeeds, astronomers can count on seeing better and better images of the heavens from earth's surface.