Design, fabrication, and testing of lightweight silicon mirrors

This paper describes the design, fabrication and cryogenic testing of lightweight silicon mirrors. Silicon offers significant advantages over other optical substrate materials. It has superior thermal properties at cryogenic temperatures, is quickly and inexpensively super-polishable, and is comparably lightweight to other substrates such as beryllium, silicon carbide, graphite epoxy and carbon. both bonded single crystal silicon (SCSi) and lightweight composite mirrors have been produced. The bonded mirrors were fabricated from two SCSi half disks joined using a proprietary process to form a slightly oblong three-inch diameter mirror. The nominally three-inch diameter composite mirrors consists of a silicon foam core, with SCSi faceplates bonded to the front and back surfaces. All optics were tested at the JPL's Cryogenic Test Facility. They were mounted in a custom and proprietary OFHC copper test fixture designed to prevent figure loss over the range of test temperature, and provide excellent thermal contact with the cryogenic chamber's cold plate. Results so far indicate that the bonded mirrors maintain their ambient figure at temperatures down to -183 degrees C.