An Observation of Pit Formation in a Multilayer Dielectric Solar Rejection Filter as a Result of Simulated Space Environmental Exposure

摘要

A common design of spaceborn telescopes involves optical elements that are directly exposed to the space environment. In one particular design, the exposed element is a semiconducting lens that was coated with a thick dielectric stack for the purpose of solar rejection. There is some concern that exposure to the energetic electrons associated with the space environment will charge this coating to the point of dielectric breakdown, which may cause damage to the coating. This paper documents the observation of charge induced pitting of test coupons that were irradiated in the Aerospace Corporation's space environmental effects simulator. Optical and scanning electron microscopy revealed extensive pitting as a result of this exposure. It was observed that the typical size of discharge pits was 50-100 microns at the surface, extending to the substrate material, where a 10 micron diameter melt region was found. Pitting was not observed on similar samples that had also been overcoated with a conductive thin-film. Measurement of the bi-directional reflectance transfer function (BRDF) showed that pitting caused a 5-10 fold increase in the scattering of visible light.