In-situ cleaning of the primary mirror of Subaru telescope

The factors degrading optical performance of the telescope optics are the deterioration of the reflecting coating itself on the surface, and the accumulation of the contamination on the mirror surface. We consider that fine cinders at the Mama Kea summit are blown into the enclosure by wind and get stuck on the optical elements, particularly on the primary mirror as it has large area and it will be looking up the sky for long hours during its operation. Contamination on the primary mirror surface decreases its reflectivity and increases its emissivity and scattering. These will affect the observational efficiency of the Subaru telescope, for it will be used in the wide range of the wavelength. Not only the decrease in the reflectivity but also the increase in the emissivity are of major concern in the infrared region. In order to prevent the accumulation of the contaminant particles, which cleaning technique could be applicable for the large telescope optics ? Several cleaning methods to replace freon cashing are devised to clean silicon wafer in its production process in the semiconductor industries. We adopted basic concept from such techniques and made experiments in the hope of using for the preventive maintenance of the telescope mirrors.