Two methods of ion cleaning of contaminant films from spacecraft optics while on orbit are examined. The first method uses energetic electron sputtering, and the second uses low-energy oxygen ion (LEOI) reactive etching. Water, ammonia, and carbon dioxide cryofilms with electron sputtering have been successfully removed. The mirror materials being cleaned include bare beryllium and protected aluminum coated optics. No damage to these ultralow-scatter mirror materials has been observed, even when the mirrors are purposely overcleaned with electrons by any orders of magnitude. Although electron cleaning sometimes polymerizes or carbonizes organic contamination instead of removing it, LEOIs are able to clean both polymerized and unpolymerized organic material at room temperature without causing damage.
Bi-directional Reflection Distribution Function measurements were performed as a function of cryogenic temperature for various substrates. Substrates investigated include HIPed and sputtered beryllium produced from different powders and by various manufacturing and polishing processes. In some samples investigated, the BRDF at 10.6 microns increased by a factor of 2 to 5 during cooling from 300 to 30 Kelvin. On repeated temperature cycling the change in BRDF appeared to be totally elastic. The cryo-scatter effect does not occur for all types of beryllium.
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