<title>Design, fabrication, and test of a meter-class reaction bonded SiC mirror blank</title>

Abstract

The potential for using silicon carbide as an optical substrate has been recognized for a number of years. The primary characteristics silicon carbide offers relative to other more traditional materials include high stiffness, high toughness, low toxicity, low thermal distortion and potential cost and schedule advantages. These attractive properties become accentuated as the size of the mirror blank increases, especially when considering space borne applications. In this paper, we report on the continuing development of silicon carbide for use in large lightweight mirror applications. In particular we describe the design, fabrication and testing of a 0.8 multiplied by 1.1 meter open backed, 'egg-crate' reaction bonded silicon carbide mirror blank. Several process demonstration (sub-scale blank fabrication repeatability and optical finishing) and material evaluation (coefficient of thermal expansion, modulus of rupture, and chemistry) tasks were performed to initiate a database for future optical designs and evaluation. Finally, the results of cryogenic testing of the blank are presented. All of the silicon carbide blank fabrication work in support of this program was performed by Carborundum Specialty Products. The results indicate that reaction bonded silicon carbide is an excellent material for large lightweight mirror substrates.