Vacuum BRDF Measurements Of Cryogenic Optical Surfaces

Spaceborne optical systems designed for long-duration missions are susceptible to degradation from contaminants in the spacecraft environment. In order to assess contamination build-up rates and corresponding scatter changes, we have designed a BRDF measurement system for use in a high-vacuum (10-8 Torr), low-temperature (20K) environment. The system measures cryocooled optics at angles from near-specular (< 1°) to approximately 30° off-axis, at wavelengths of 0.633 and 10.6 microns. We describe the design characteristics of the instrument, and discuss preliminary experiments which demonstrate contamination build-up correlated with changes in the sample BRDF profiles. The current status of the instrument is reviewed and future plans and applications summarized.