Abstract
Airborne contaminations on telescope mirrors are an important issue that is significantly affecting their reflectance, IR emissivity, and light scattering properties. Microscopic damage caused by environmental contaminants is of major interest for high-performance telescope mirror coatings. We have exposed unprotected Al coated mirrors under real operation conditions at the Very Large Telescope on Cerro Paranal/Chile for three years. The unique finding is that, in spite of a dry and low-dust environment, the reflective layers are damaged by interaction between dust particles on the Al mirror surface and capillary condensation. We analyze this particular damage mechanism at the microstructural level.
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