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Challenges in high resolution space telescopes have led to the desire to create large primary mirror apertures. Ceramic mirrors and complex structures are becoming more important for high precision lightweight optical applications in adverse environments. Carbon-fiber reinforced silicon carbide (C/SiC) has shown great potential to be used as mirror substrate. This material has a high stiffness to weight ratio, dimensional stability from ambient to cryo temperatures, and thermal conductivity, low thermal expansion as well. These properties make C/SiC very attractive for a variety of applications in precision optical structures, especially when considering space-borne application. In this paper, lightweight C/SiC mirror prepared for a scan mirror of a high resolution camera is presented. The manufacturing of C/SiC mirror starts with a porous rigid felt made of short chopped carbon fibers. The fibers are molded with phenolic resin under pressure to form a carbon fiber reinforced plastic blank, followed by a pyrolization process by which the phenolic resin reacts to a carbon matrix. The C/C-felt can be machined by standard computer controlled milling techniques to any virtual shape. This is one of the most significant advantages of this material, as it drastically reduces the making costs and enables the manufacture of truly ultra-lightweight mirrors, reflectors and structures. Upon completion of milling, the C/C-felt preform is mounted in a high-temperature furnace together with silicon and heated under vacuum condition to 1500°C at which the silicon changes into liquid phase. Subsequently, the molten silicon is infiltrated into the porous preform under capillary forces to react with carbon matrix and the surfaces of the carbon fibers to form a density C/SiC substrate. The C/SiC material retains the preform shape to within a tight tolerance after sintering means the ceramization process is a nearly net shaping process. Reactive melt infiltrated C/SiC, followed by chemical vapor deposited silicon carbide (CVD SiC) cladding, is used to fabricate a 225-mm×165-mm ellipse mirror 18-mm thick and 0.41 kg weight. The mirror's backing structure contains hexagon "pockets". The individual ribs are only 2-mm thick, each contains a large cutout for structural efficiency and improves the mirror's thermal properties. Open back honeycomb lightweight structure is produced to gain 14kg/m2 area density. CVD SiC has an excellent adherence to C/SiC. It also has an excellent thermal strain match. Approximately 150-μm of this material is used to clad the mirror substrate. The CVD SiC cladding is polished to be a super smooth surface with less than 0.372-nm RMS surface roughness. Experimental results indicate that reactive melt infiltrated C/SiC can be used as optical mirror substrates. Currently, experiments are under way to fabricate a large-scale lightweight C/SiC optical mirror in diameter larger than 600-mm.
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