Word: mirrored
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Dates: during 1990-1999
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...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...
Between this Scylla and Charybdis, mirror designers are charting a variety of bold, new courses. By designing the Keck Telescope mirror as a mosaic of small segments, each the size of a dining-room table, astronomer Jerry Nelson of the University of California, Berkeley was able to make his mirrors both rigid and thin. But to provide images of pinprick sharpness, each segment must be kept perfectly aligned with its neighbors, a task handled by an elaborate electronic network...
...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...
...contrast, the mirrors designed for the European Southern Observatory consist of a single, vast expanse of glass, thin (17.7 cm) and very flexible. To control wobbling and stabilize the orientation, these mirrors, like giant catcher's mitts, will be constantly readjusted by 180 computer-activated steel "fingers." A prototype mirror has already proved its worth. A flaw identical to the one that crippled the Hubble Space Telescope was easily corrected by adjusting the mirror's shape...
Angel's approach relies less on intricate control systems and more on vitreous wizardry. The 10-ton mirror he and his colleagues plan to install in Arizona -- merely a warm-up for some 8-m versions -- boasts a light-collecting surface that is nearly as wide as a house is tall, yet it averages only 2.8 cm thick. What prevents this marvel from fracturing under its own weight is a supporting truss composed of thousands of glass ribs that are cast as part of the mirror's underlying structure. Arrayed in a striking hexagonal pattern, the ribs form an airy...