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

Yong Shu ,Yifan Dai ,Zhong Zheng ,Shengyi Li

doi:10.3844/ajnsp.2010.45.50

Abstract

Problem statement: Silicon Carbide (SiC) optical materials have becom e the first choice for mirrors in space optical systems and large grou nd-based optical systems due to their outstanding mechanical, physical and optical properties. Compar ed 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 ultr a-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 resul ting 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 met hod 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 0.175 λ(PV)/0.009 λ(RMS) from 0.526 λ(PV)/0.080 λ(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.

Highlights

The RB Silicon Carbide (SiC) has an lead to deformation which challenges the precision of advantage over other kinds of SiC that it can be mirror and result in a rising cost of launching it to fabricated in nearly any kinds of configurations
SiC material, experiments are conducted under the material removal amount, removal function and conditions listing in Table 1 to get the rules between resident time can be written as: polishing parameters and resulting surface roughness
The workpiece is Computer Controlled Optical Surfacing (CCOS) result: A 475mm diameter sphere RB SiC mirror was polished on our AOCMT (Aspheric Optical Compound Machining Tool)

Summary

INTRODUCTION

The material used for these mirrors should have following properties such as. Along with the development of space optical relative low density, high specific stiffness, great heat systems and large ground-based optical systems such as conductivity, appropriate strength and low hardness. SiC optical material has become a very promising satellite, meteorological satellite, super laser and laser material for large reflector due to its outstanding radar, the diameters of mirror in these optical systems properties. For space RB SiC optical material with pitch and diamond optical system, mirror will face a more rigorous polishing powder (Kang et al, 2008) can get good environment such as ultra-low temperature, stronger surface roughness. Our goal is to employ CCOS for the high precision fabrication of RB SiC mirrors

MATERIALS AND METHODS

DISCUSSION

CONCLUSION