Low cost, high performance lightweight Silicon Carbide (SiC) mirrors provide an alternative to Beryllium mirrors. A Trex Enterprises 0.25m diameter low areal density SiC mirror using its patented Chemical Vapor Composites (CVC) technology was evaluated for its optical performance at cryogenic temperature. CVC SiC is chemically pure, thermally stable, and mechanically stiff. CVC technology yields higher growth rate than that of CVD SiC. NASA has funded lightweight optical materials technology development efforts for future space based telescope programs. As part of these efforts, a Trex SiC mirror was measured interferometrically from room temperature to 30 degrees Kelvin. This paper will discuss the test goals, the cryogenic optical testing infrastructure and instrumentation at MSFC, test results, and lessons learned.
The chemical vapor composites (CVC) process provides for the rapid manufacture of near net shape, reduced residual stress silicon carbide (SiC) suitable for high performance optics. The reduction or elimination of residual stress provides several key advantages: 1) increased growth rate, 2) high yields, and 3) near net shape deposition of complex geometries. Near net shape deposition allows for fabrication of spherical and aspherical optics without machining of the optical surface. Final surface figures of optical flats are typically better than 1/10λ (P-V) and 2-5Å surface roughness. A comparison of ultraviolet spectrum reflectance of CVC SiC and that of single crystal SiC is discussed. The complex optical constants of CVC SiC in the mid-infrared spectrum are also presented.
The demand for high performance lightweight mirrors has never been greater. The coming years will require lighter and higher performance mirrors and in greater numbers than is currently available. Applications include both ground and space based telescopes, surveillance, navigation, guidance, and tracking and control systems. For instance, the total requirement for US government sponsored systems alone is projected to be greater than 200 m2/year1. Given that the total current global production capacity is on the order of 50 m2/year1, the need and opportunity to rapidly produce high quality optics is readily apparent. Key areas of concern for all these programs are not only the mission critical optical performance metrics, but also the ability to meet the timeline for deployment. As such, any potential reduction in the long lead times for manufactured optical systems and components is critical. The associated improvements with such advancements would lead to reductions in schedule and acquisition cost, as well as increased performance. Trex’s patented CVC SiC process is capable of rapidly producing high performance SiC optics for any optical system. This paper will summarize the CVC SiC production process and the current optical performance levels, as well as future areas of work.