The Ultra-Precision Polishing of Large Aperture Reaction Bonded Silicon Carbide Mirror

Problem statement: Silicon Carbide (SiC) optical materials have become the first choice for mirrors in space optical systems and large ground-based optical systems due to their outstanding mechanical, physical and optical properties. Compared with traditional optical glasses, SiC optical materials are provided with the characteristics of high hardness and multiphase, which embarrass the high efficient fabrication of SiC mirrors with ultra-smooth surfaces and high precision. Approach: We choose some typical polishing parameters and conduct a series of experiments, trying to find out the relationship between polishing parameters and resulting surface roughness. The polishing parameters for Reaction Bonded Silicon Carbide (RB-SiC) are optimized from the analysis of these experiments. Then we can apply these parameters to the figuring process. Computer Controlled Optical Surfacing (CCOS) is a widely used deterministic polishing method and features in low-cost, high-precision and large flexibility. The basic theory and process of CCOS are given in detail. Then we employ CCOS in the ultra-precision machining of RB mirrors. Results: We polish a RB SiC sample with the optimized polishing parameters and a surface roughness better than 1nm (RMS) is obtained. A 475 mm diameter sphere RB-SiC mirror with a relative aperture of 1:1 is polished by CCOS and the surface error reduces to 0.175λ(PV)/0.009λ(RMS) from 0.526λ(RMS). Both results represent a very smooth surface and a very precise figure. Conclusion: The result proves the feasibility of the polishing technology and CCOS method. These works also accumulate experience for the manufacturing of aspheric SiC mirror.