Feature

Aspheric polishing with a stressed lap

H.M. Martin

The 1990s will see the construction of six to eight optical telescopes with effective diameters in the range 6.5 m to 16m, all larger than any existing telescope. The huge increases in light-gathering power have come about only through innovative mirror and telescope designs, for gravity does not allow a simple scaling up of existing 4-m-class telescopes. Thus, three new primary mirror concepts are being pursued vigorously: the segmented mirror of the 10m Keck Telescope,1 which consists of 36 thin 1.8-m segments of solid low-expansion glass ceramic; monolithic thin meniscus mirrors of the same material for the European Southern Observatory's 4 x 8.2-m Very Large Telescope2 and the 7.5 m Japanese National Large Telescope; and spin-cast borosilicate honeycomb mirrors up to 8 m in diameter for three projects involving the University of Arizona and two U.S. national telescope projects.