Optical Surface Degradation from Combined Ultraviolet Radiation and Outgassed Materials

Abstract

Among problems, which have been encountered in testing and flight of spacecraft, have been those related to the outgassing of materials in vacuum. Two of the major problems caused by outgassing are electrical corona discharge, and condensation onto critical surfaces. The surfaces of a spacecraft most susceptible to damage by condensation of a foreign material are those used for thermal control and those used in optical systems. This paper describes the techniques used to determine the effects of outgassed materials condensed on MgF2 over-coated aluminum mirrors while the mirrors were irradiated with ultraviolet light in vacuum. Mirror temperatures as low as −60 °C were used to determine the point at which condensation was incipient. The measurements performed on these mirrors included reflectance at Lyman α (1216 Å) and infrared analysis of the deposit. The initial results obtained with this system demonstrate that Lyman α reflectance is essentially unaffected by heavy condensates of some materials but almost completely destroyed by very thin deposits of other materials.