Laser-mirror cleaning in a simulated space environment

The efficacy of pulsed CO2 lasers is demonstrated for remote removal of contaminants from cryogenic, low-scatter mirrors in a simulated space environment. A gold-overcoated, nickel-coated aluminum mirror was housed in a vacuum dewar and cooled to either 90 K with liquid nitrogen or 34 K with liquid-helium cryogen. Admitting a controlled leak of room-air contaminated the mirror. Bidirectionl reflectance distribution function (BRDF) measurements at 3, 6, and 9 deg monitored the amount of contamination on the mirror. Upon contamination, the mirror BRDF generally increased by an order of magnitude or more. Using an x-y scanner to raster a pulsed CO2-laser beam across the face of the mirror, successful cleaning of sizable areas (25 sq cm) of the mirror was demonstrated at both temperatures. In all cases the laser-cleaning technique returned the BRDF of the mirror to precontamination levels and showed no evidence for contaminant redeposition within the cleaned area.