Science: Better Spyglass on the Stars

A scheme for the world's biggest optical telescope

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The towering Mauna Kea, a 13,800-ft. extinct volcano in Hawaii, is a peculiar mix of the exotic. Gnarled koa trees twist up from its tropical slopes, where the endangered palila bird, a tiny yellow honey creeper, crushes rock-hard mamane seeds with its beak. But up on top, science has taken over. Because the exceptionally dry and stable atmosphere over Mauna Kea makes the site among the world's best spots for star gazing, six telescopes have been built on the volcano's crest, and two more are under construction.

Now an extraordinary newcomer will join that celestial company. The California Institute of Technology, working with the University of California, will build the world's biggest optical telescope on the volcano's crest; construction could begin as early as 1986. The mammoth instrument, made possible by a $70 million grant to Caltech by the W.M. Keck Foundation, will have an innovative mirror system nearly 400 in. in diameter, which is twice the width and has four times the light-gathering capacity of today's reigning optical telescope, the 200-in. Hale device at Mount Palomar, Calif. When astronomers begin using the new telescope in 1992, it will push back the visible limits of the universe by billions of light years. Says Howard Keck, president of the foundation: "I'm told it will permit one to see the light of a single candle from the distance of the moon."

The Keck telescope, as it will be called, is the first of a new generation of extra-large telescopes designed to overcome construction problems that have dogged sky gazers since before the Hale was dedicated in 1948. Photons (massless particles that transmit light) from an ancient galaxy may travel billions of light years through space before they speed down a telescope tube. But unless enough of them are collected, astronomers will not be able to see the galaxy's image. Gathering sufficient photons to register an image is accomplished by either taking long-exposure photographs or using a larger mirror system to collect the light. Many astronomical photographs already take hours to make, but even then not enough photons can be gathered for a clear view of very faint objects. Hence the need for bigger mirrors. Complains Palomar Observatory Director Gerry Neugebauer: "We're photon starved."

Yet conventional telescopes cannot be simply scaled up indefinitely. As the mirrors get bigger, they begin to sag under their own weight. Indeed, for years many astronomers believed that a 200-in. diameter was the practical limit for an optical telescope.

The advent of the computer has changed all that. Led by Astrophysicist Jerry Nelson, a team at the University of California designed an unorthodox mirror that will not be a continuous concave surface, like Hale's, but 36 hexagonal pieces of specially shaped glass, each 6 ft. across and 3 in. thick; the segments will be fit together and will move in concert to act as one giant parabolic mirror. That harmony is possible only with the aid of a computer- controlled sensing and positioning system, which will realign the components 100 times a second by as little as one one-thousandth the width of a human hair.

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