Big Telescopes on a Roll: The World Has a Number of Projects for Telescopes in a Size Range Thought to Be Impossible Not Many Years Ago

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Big Telescopes on a Roll

The first astronomical telescope was two lenses in a tube; Galileo could hold it in his hand. Today's telescopes are so big that mountaintops sometimes have to be sheared off to make room for them, and they are getting even bigger.

Until about a decade ago, astronomers thought they had reached the practical limit of size with telescope mirrors of 5 or 6 meters diameter. Now, thanks to technological developments in the construction and management of large mirrors, these limits are being surpassed, even doubled. The world now has approximately 10 projects in different stages of construction, planning or discussion that intend to use mirrors larger than 6 meters.

Glass was the greatest hindrance to building large mirrors. To image the sky properly, a telescope mirror must keep the shape of its curved surface precise. The necessary stiffness seemed to require a thick backing of glass behind the reflecting surface. As diameters got up to 6 meters or more, a catch-22 came in: In large telescope mirrors the amount of glass seemingly required for stiffness would slump under its own weight and distort the reflecting surface it was supposed to maintain.

Two solutions are actively being tried. One stops relying on the glass for stiffness. Such plans envision a segmented mirror with what telescope designers call "active support,' an arrangement of levers and thrusters that holds the overall shape of the mirror, compensating for gravity, wind stress and other distorting factors.

The second solution is still "passive.' It relies on the stiffness of the glass but aims to prevent slumping by leaving out most of the glass. It turns out that this is possible if the back of the mirror is in the form of a proper kind of honeycomb shape rather than a solid slab.

Such honeycombing seems first to have been tried with the 5-meter mirror on Palomar Mountain. J. Roger Angel and his associates at the University of Arizona in Tucson have developed it into a production-line technique, in which plugs of water-soluble material set in the bottom of the mold make the voids in the honeycomb and are then washed out of the finished mirror with a high-pressure stream of water.

A major innovation of Angel's group is to use a rotating mold. Rotation gives the upper surface of the telescope blank a paraboloidal surface instead of the flat surface of ordinary casting. A paraboloid is the surface shape most telescope designers want, and so starting with it greatly simplifies the grinding and polishing of the surface. For a large conventional mirror, shaping and grinding take years, and the time and expense of this step posed another hindrance to planning mirrors bigger than 5 meters.

The recent developments have meant, among other things, that two people are having significant influences on an entire generation of new telescopes. Nearly all the current large projects have had Harland Epps of the University of California at Los Angeles involved in the design of their optics, and most of them will want mirrors of the type Angel casts.

Angel's group started out with a 2-meter rotating furnace. Having successfully cast 2-meter mirrors, they began about three years ago to build a new casting shop under the stands of the football stadium on the Tucson campus with the intention of ultimately casting 8-meter mirrors (SN: 2/16/85, p.106; 1/17/87, p.40). A session on the status of the large-telescope projects at the recent Workshop on Instrumentation for Groundbased Optical Astronomy, held at the University of California at Santa Cruz, heard from John Hill of the University of Arizona that the 8-meter turntable has just been completed. It now carries a furnace of 3.5 meters diameter for its first project, a mirror of that size for the projected ARC telescope, expected to be cast in November.

Later Angel and his colleagues expect to cast another 3. …