Progress toward making lightweight 8 m mirrors of short focal length

The ideal 8 m primary mirror for astronomical telescopes should be rigid, lightweight, thermally responsive and have short focal length. To satisfy this need, a technique for casting glass in the form of honeycomb structure has been developed at the Steward Observatory Mirror Lab. This form is preferred for stiffness and light weight, and also lends itself to thermal control, by forced ventilation of the honeycomb cells. The fabrication method has been proven by the successful casting of three blanks of intermediate size, 3.5 m diameter. The furnace is now being increased in size to accomodate 8 m castings. It is expected that the first large blank, of diameter 6.5 meters, will be cast in February, 1991, and the first 8 m blank a year later. No mirror as large as 8 m has ever been polished. The challenge is not only the very large size, but also the higher accuracy levels and shorter focal ratios now demanded for telescope optics. A new technique, stressed lap polishing, has been developed to meet this challenge. The 8 m mirrors will be polished in a newly built extension to the Mirror Laboratory that will house an existing 8 m vertical lathe for numerically controlled diamond grinding, and a new 8 m polishing machine to be tailored for the stressed lap method.