Silicon Carbide Lightweight Optics With Hybrid Skins for Large Cryo Telescopes

Optical Physics Company (OPC) proposes to manufacture new silicon carbide (SiC) foam-based optics that are composite, athermal and lightweight (FOCAL) that provide an enabling capability for performing NASA space missions that will require 2 to 3 meter class cryogenic mirrors for large infrared telescopes. Open-cell foam core mirror technology has evolved over the past ten years and has produced somewhat large (0.5 m dia.), lightweight (<10 kg/m2), rapidly and uniformly cryo-cooled and dimensionally stable (tested to 25 K) mirrors configured with monolithic chemical vapor deposited (CVD) skins. Significant problems have arisen as the size has increased that prevent their use on large aperture telescopes. These problems include the inherent stress in the monolithic skins which can result in skin cracking during the substrate manufacturing and finishing processes, the non-uniformity of the .040"-.050" thick monolithic skins that typically require .010"-.015" of material removal before a continuous surface can be achieved for optical finishing, the long schedule of manufacturing the mirror substrate, and the large $2M/m2 cost to produce the polished mirror. OPC proposes an innovation where the monolithic SiC skins are replaced with SiC fiber reinforced/SiC CVD skins. This hybrid design consists of a .020"-.030" thick SiC fiber reinforced layer ground to a smooth finish and near net shape, and then post coated with a .005"-.010" thick CVD SiC polishing layer. This provides increased skin strength and toughness to enable the technology to produce meter class mirrors without skin cracking. The manufacturing time and CVD chamber cost are reduced because premanufactured SiC fibers are used to provide the bulk of the skin mass rather than laying down a monolithic skin atom by atom via CVD. The net effect is to produce a SiC FOCAL mirror substrate that is stronger, tougher, scalable to meter class, and potentially better than 50% faster and cheaper to manufacture.