Current Status of the MMT Optics

Abstract

The Multiple Mirror Telescope, a joint University of Arizona and Smithsonian Astrophysical Observatory project, was first reported in this journal in 1972. The optics are in production, some complete, others near completion. Some of the ideas we used worked, others did not. This paper discusses the techniques actually used and how well they succeeded. The Multiple Mirror Telescope i(MMT) was described previously in the March-April 1972 issue of this journal. For a quick review, Figs. 1 and 2 present the optical design of this instrument. It consists of six independent Cassegrainian telescopes placed in one optical support structure (OSS) and mounted in an altitude over azimuth mounting. The six telescopes will be collimated to a mutual axis and maintained by the use of an active optical alignment system. The alignment signals, as well as stellar tracking information, are gen erated by a seventh (30 diameter) guide-alignment telescope located in the center of the large array. Each of the Cassegrain telescopes has a primary aperture, 72 in diam-eter, and either a 9.5 or 10.5 diameter secondary. The reason for two secondary mirror diameters is that the telescope will be used in both the IR and visible regions of the spectrum. The instrument is designed primarily for the 1R, but differences in secondary coatings and design requirements dictated that two independent secondary configurations be fabricated. The primary mirrors are of lightweight fused silica eggcrate construction. Each of these mirrors weighs approximately 1200 lbs. and had to be slumped from their original flat configuration to the required radius of curvature of 388.8. When placed into the telescope their completed fino. will be f/2.7 yielding a total telescope Casse-grain f/no. of f/31.7. The total array f/no. is determined by the beam combiner (second folding flat) and will be approximately f/9 initially.