Word: mirrored
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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...
...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...
Although the conceptual design appears straightforward, the casting of a honeycomb mirror requires considerable technical know-how -- and time. Angel's team tackles the job in their hangar-like mirror lab located, improbably enough, under the stands of the University of Arizona football stadium. In the center of the lab is a huge round furnace. To make a mirror, a complex ceramic mold is assembled inside the furnace and filled with glittering chunks of Pyrex-type glass. Once the furnace lid is sealed, the temperature will slowly ratchet up over a period of several days, at times rising no more...
Cooling the mirror is an equally painstaking process that takes many weeks. Reason: if one section of the glass cools faster than another, it will contract more quickly, creating stresses that lead to cracking. When finally unmolded, the mirror will still require months of tedious polishing to remove any imperfections...