New telescope for old.

New telescope for old

When the Multiple Mirror Telescope (MMT) was completednine years ago, astronomers waited eagerly to see whether this most innovative of telescopes would really work. The MMT consists of six mirrors set on a single mount; among the questions were whether such a seemingly unwieldy thing could track and point accurately, and whether six mirrors could be made to throw their reflections into a single image.

The MMT's managers believe it has fulfilled all those expectationsvery well. But Frederick H. Chaffee Jr. of the MMT Observatory in Amado, Ariz., told the meeting that this does not deter them from planning to replace its six mirrors with a more conventional single one. The MMT can point to within 0.3 second of a arc, it can track objects across the sky to an accuracy of 0.1 second of arc, and after a great deal of work, its images can be coaligned and made to coincide as well as astronomers would like. Indeed, experience with the MMT has led the designers of the National New Technology Telescope to choose a multiple mirror design for that project.

In spite of the success, the six 1.8-meter mirrors of the MMThave the total light-gathering power of a single mirror of 4.5 meters diameter. When the MMT was planned, that was a large power; today, with people planning 8-, 10- and even 16-meter equivalents, it is rather ordinary. So the MMT's managers have decided to upgrade their capacity by replacing their six mirrors with a single 6.5-meter mirror to be mounted in the same frame that now holds the six.

Casting single mirrors of this size was technologically impractical when the MMT was planned. Now, thanks to development of a method of casting very light, large, single mirrors in a spinning furnace, pioneered by Roger Angel of the University of Arizona, single mirrors up to 8 meters across are possible. The MMT people expect Angel to cast their mirror in 1988, and they hope to complete the conversion by 1922.