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Study on Ultraprecision Polishing of Sapphire – Effects of Crystal Orientation on Polishing Characteristics

In the telescopic interferometer KAGRA (Large-scale Cryogenic Gravitational wave Telescope), the ultraprecision mirror made of sapphire (α-alumina) is required to polish. The focal length of the mirror is 7 km and it must be finished by polishing after grinding. In order to polish the mirror precisely, polishing characteristics of the sapphire must be researched. In this study, the polishing characteristics are examined and the dependence of the crystal orientation on the polishing rates is studied. In the experiments, sapphire wafer is polished by using rotational small tool and SiO2 abrasives.