Nitrogen snow cleaning inside a large cryogenic telescope

摘要

A 50-cm cryogenic mirror at one end of an aluminum telescope was successfully cleaned by nitrogen snow in a series of demonstration tests. The mirror was maintained below 70 Kelvin under vacuum during the cleaning, with a 15 Kelvin cold cap pumping the nitrogen gas to maintain a realistic space environment. The snow was produced by an assembly of 6 nozzles and valves attached to the exterior of the telescope. The nozzles protruded less than 1 cm into the telescope and were well outside the mirror diameter. Contamination of the mirror was produced by silica and alumina dusts propelled into the telescope by special velocity-moderating sources. Cleaning effectiveness was measured by scatter of 10.6-mu laser light at 2 degrees from three spots on the mirror surface. All scatter system components were exterior to the telescope, with only small holes for the passage of laser radiation. The clean mirror BRDF of 5 multiplied by 10$+$MIN@4$/ sr$+$MIN@1$/ was raised as high as 3 multiplied by 10$+$MIN@2$/ sr$+$MIN@1$/ by the contamination process and subsequently reduced to the original level by one or two seconds of nitrogen snow spraying. Nitrogen snow cleaning under vacuum proved much more effective than carbon-dioxide snow cleaning of the same mirror in air.