ELID grinding of SiC ultra lightweight mirror

Abstract

Silicon carbide (SiC) is the most advantageous as the material of various telescope mirrors, because SiC has high stiffness, high thermal conductivity, low thermal expansion and low density and so on [1] [2]. However, since SiC is very hard, it is difficult to obtain the surfaces efficiently by using ordinary grinding method. Therefore, we developed the ultra lightweight mirror by ELID (ELectrolytic In-process Dressing) grinding method and ultra-precision grinding machine. ELID grinding method is the new grinding technology which one of authors invented, produces the quality of the high surface efficiently, and is effective in processing of a very hard material, such as crystalline silicon and sapphire, ceramics, glasses, hard metals and so on. SiC mirror for space telescope is designed as regular polygon rib structures in the rear face for reducing weight, and both the optical face and the backside ribs are so thin that deformation of the mirror cannot disregard. Therefore, in order to feed back deformation of the mirror to the orbit of grinding wheel, it is necessary to analyze deformation. In this research, we present the results which fabricated mirrors of sintered SiC by the ELID grinding method, the numerical calculation results of the profile deformations analyzed by FEM, and new jig for Φ360mm a mirror.