Optical system contamination: formation of films and droplets

Abstract

Molecular film contamination is known to degrade the optical performance of space system components, including solar arrays and thermo-optical, second surface mirrors. In the form of a contaminant film, the resulting performance loss rate can be evaluated using traditional models for absorption and reflection. In recent years, however, some space-borne optical sensors have suffered severe and rapid performance degradation due to the formation of contaminant droplets that fog interior lenses, mirrors, and windows. Optical system analysts tasked with predicting the loss of throughput due to molecular film contamination have not addressed the impact of droplets in great depth. This paper investigates the conditions leading to the formation of films or droplets resulting from the outgassing products of typical spacecraft materials. A simplified view of surface energy and the wetting parameter are used to show that typical outgassed contaminants and optical substrates favor the formation of droplets. Therefore, analysis of throughput losses due to the scattering of droplets is critical. The droplets can be converted into films or extended islands when exposed to vacuum ultraviolet (VUV) radiation. This observation shows why droplets are rarely observed on external thermal control mirrors and solar arrays but might be considered highly likely in a low-VUV environment.