Spinning a large telescope from glass.

Spinning a large telescope from glass

In the Atacama Desert of northern Chile lies one of the world's major concentrations of astronomical observatories. There, if plans announced last week by the University of Arizona in Tucson, the Carnegie Institution of Washington, D.C., and the Johns Hopkins University in Baltimore are carried through, Carnegie's site at Las Campanas will get an 8-meter-diameter telescope. This will be the largest in the Southern Hemisphere.

The southern skies are as interesting -- in certain cases more interesting -- to astronomers as the northern skies. But due to lack of land, people and economic development in the Southern Hemisphere, telescopes there have been far less numerous and smaller than those in the north. Four meters is a large as telescopes get in the Southern Hemisphere, and the largest now at Las Campanas is 2.5 meters. The Northern Hemisphere now has a 5-meter telescope (on Palomar Mountain in California) and a 6-meter telescope (on Mt. smirodriki in the Crimea). A 10-meter telescope, the Keck Telescope of California Institute of Technology and the University Of California, is under construction in Hawaii.

The 8-meter mirror for Las Campanas will be cast by a new technique developed by J. Roger Angel of the University of Arizona, in which the casting is done in a rotating furnace (SN: 2/16/85, p. 106). The spin gives the mirror a parabolic surface that lessens the amount of grinding to be done in the finishing process and the amount of glass necessary for the casting. The back of the mirror blank is in the form of a honeycomb rather than being solid. This, too, lessens the amount of glass, the expense and the weight, all of which have been limits on the size of single large telescopes.

Eight meters is the size of the large casting machine now being built by Angel and his collaborators under the stands of the University of Arizona football stadium in Tucson. If some other current plans work out, the same furnace will cast two 8-meter mirrors that the University of Arizona, Ohio State University, the University of Chicago and a fourth, unnamed, partner plan to put on Mt. Graham, near Willcox, Ariz. Angel's method has been used successfully for smaller mirrors, especially a 1.8-meter mirror for the Vatican Observatory located in Castel Gandolfo, Italy.

How these smaller mirrors work out will affect the final decision of the institutions involved in thenew southern telescope. So far, they are committed only to the design. A later review will determine whether construction goes ahead.