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.
Molecular contamination degrades sensitive spacecraft surfaces and can adversely affect the useful life of a spacecraft.
In order to accurately predict spacecraft performance and end of life, an understanding of the primary mechanisms and
processes involved in the deposition and "fixing" of molecular contaminants is necessary. The objective for this research
effort has been to investigate how solar vacuum ultraviolet (VUV) radiation and surface temperature influence
photochemical reactions of molecular contaminants. This report presents the effects of VUV intensity and surface
temperature on photo-deposition and "photo-fixing" of dioctyl phthalate (DOP) films.
Particle-induced light scatter reduces sensitivity and degrades performance of optical systems. Though particles are generally considered to be the primary source responsible for stray radiation, there is evidence that molecular contaminants also induce light scatter. The primary objective for this research effort has been to increase our understanding of molecular contaminant film growth and its implications for light scatter. Herein, our new molecular film deposition and imaging facility is described. In-situ imaging data, acquired from non-uniform films of contaminant analogues, has revealed that even small quantities (less than 100 angstrom mass equivalent) of molecular contamination can produce scattered light. These data suggest that small amounts of molecular contamination have the potential to significantly impact the performance of scatter sensitive optical systems.