Mirror cleaning and reflectivity degradation at 1300 and 2300 m above sea level at Mt. Hopkins, Arizona

The factors degrading optical performance of the atmospheric Cherenkov telescope optics are the deterioration of the reflecting coating itself on the surface and the accumulation of the contamination on the mirror surface. After an exposure of more than two and half years in the open air the mirrors cleaned monthly by just water were not subject to any considerable degradation in their reflectivity. The only drop in the reflectivity (∼5%) was at 310 nm. The optical maintenance of the Whipple mirrors and evaluation with test mirrors at the two potential sites (1300 and 2300 m above sea level) at Mt. Hopkins are reported. Generally, the results at both sites are comparable. Sunlight seems to be an important parameter leading to mirror degradation either by itself or together with contaminants. The various cleaning methods used provided almost the same reflectivity restoration with somewhat better performance for mirrors at 1300 m asl. The present results suggest that mirror cleaning can minimize the down time for coating renewal. This is particularly important for the VERITAS project and other large arrays currently under construction.