Aluminum Mirror Coatings in Space: A Study of the Decrease in Ultraviolet Normal Incidence Reflectance Produced by Controlled Oxidation of Evaporated Aluminum Mirror Surfaces

Pure aluminum is an excellent normal-incidence reflector at wavelengths longward of 90 nm. However in the terrestrial environment, a fresh surface undergoes rapid oxidation, which leads to severe reflectivity losses at wavelengths below 180 nm. Results of an experimental study indicate that in a spacecraft spectrometer constructed to conventional UHV standards, we anticipate an exposure of 10**4 L in one year can be anticipated which would reduce the 121. 6 nm reflectivity to 70% and the spectrometer throughput to 50%. The loss at 90 nm would be more severe. This represents a performance within the design requirements of the mission, but one which could clearly be improved considerably if the overall vacuum environment could be improved, for example by using large gettering areas on the walls of the spectrometer enclosure.