Shoot for the Stars

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TWELVE SUMMERS AGO, UNIVERSITY of Arizona astronomer Roger Angel swung by a Tucson pottery shop to pick up some firebricks for a backyard kiln. Then he purchased some glass ovenware at a nearby hardware store. A few days later, he materialized in a graduate student's doorway, brandishing a couple of Pyrex custard dishes melted to a misshapen blob. "We can make telescope mirrors out of this!" Angel exclaimed. Thus began a monumental and quixotic effort to reinvent the central light-gathering surface of the telescope, from its initial design to its final polishing.

This month, many years and millions of dollars later, that effort culminated in a spectacular success: the casting of one of the world's largest telescope mirrors, a single 6.5-m (21-ft.) circle of glass that sometime in 1994 will be hauled by flatbed truck to the top of Arizona's Mount Hopkins, where it will tilt skyward like a giant Cyclopean eye.

These are heady days in the rarefied world of telescope making. Not since the 1934 casting of Mount Palomar's 5-m mirror -- a record size at the time -- has there been more innovation or competition to push the edge of possibility. In the clear air above Hawaii's Mauna Kea, the Keck I Telescope's mammoth 10-m mirror, built of 36 separate segments, is nearing final assembly -- a 10-month process was completed last week. Four years from now it will be joined by the Keck II, an equally monstrous twin. By then, the European Southern Observatory hopes to have positioned the first of four 8.2-m telescopes atop a high peak in the Chilean Andes. Japanese astronomers and other groups around the world will be constructing telescopes of similar size and daring before the end of the century.

Collectively, this new generation of ground-based instruments will open an extraordinary new window on the cosmos. "What we can look forward to," says Caltech astronomer Maarten Schmidt, "is the biggest gain in telescope power in the past 50, maybe even 100 years." It should bring into focus the most distant quasars yet and even planets orbiting other stars.

The intellectual seeds for this technological renaissance were sown more than a decade ago, when Angel and a handful of other pioneers began contemplating the challenge of building more powerful telescopes. Very quickly, they were forced to consider radical new approaches to mirror design. Simply scaling up old models would have been hopelessly expensive and unwieldy. "A large mirror can't look like a small mirror," explains Angel, "for pretty much the same reason that an elephant can't look like a fly. If it did, its legs would collapse under its own weight."

The central conundrum confronting designers was this: how to make a telescope mirror that could hold its shape against gravitational sag and gusting winds yet retain the capacity to make rapid adjustments to fluctuating temperatures. As mirror size increases, these two requirements begin to dictate different, and quickly contradictory, solutions. Very thick mirrors resist physical deformation extremely well, but because they retain so much heat, they tend to generate shimmering currents in the cold night air that play havoc with astronomers' observations. Very thin mirrors, on the other hand, have ideal thermal properties but a daunting physical handicap: as the telescope pans across the sky, a thin mirror will bend and wobble as if made of rubber.