Large ceramic mirror for space optical applications

Lightweight ceramic composite mirrors were prepared for the large pointer mirror or the primary mirror of the high resolution camera. The Si/SiC composite was a ceramic matrix of free silicon containing a bimodal distribution of silicon carbide grains. The green body of silicon carbide grains and free carbon was fabricated by slip casting. Inserts were used to form the complex lightweight structure. During silicon vapor infiltration process, silicon reacted with carbon to form new silicon carbide phase and the pores of green body disappeared. Because there are very little shrinkage during drying and reaction process, the large scale production with complex shape has no crackles and distortions. The results indicated that Si/SiC composite could be used as space optical mirror substrates. Using electron beam physical vapor deposition, we prepared a silicon film on the mirror substrate. The properties and microstructure of Si/SiC composite and Si film were measured. We fabricated some space mirrors with various scales. A 120 mm hexagonal mirror with 15 mm thickness and 0.23 kg weight was polished and coated with Si film. Open back honeycomb lightweight structure was produced to gain 24.3 kg/m² areal density. Polishing technique different from optical glass polishing was introduced to achieve less than 0.1 λ @ 632.8 nm p-v surface error and less than 3 nm rms surface roughness. The stress and displacement under factual condition were estimated using finite element analysis method.