It has long been recognized that the quality and resolution of optical astronomical observations are limited by the disturbing effects induced during passage of light through the earth's atmosphere. For this reason, measurements have been planned to be performed on board orbiting spacecraft at altitudes where the disturbing influence of the earth’s atmosphere is negligible. Unfortunately, even in this near-vacuum environment, optical measurements can be degraded by the atmosphere induced by the spacecraft and instrumentation packages. A primary concern is the condensation and adsorption of outgas products on the various optical surfaces (lenses, mirrors, etc.) such that the optical properties of these surfaces are altered. Extremely small amounts of contamination can significantly affect the Transmissivity and reflectivity of these optical surfaces, thus seriously degrading the intended astronomical measurements.
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