Studies on evaluating and removing subsurface damage on the ground surface of CLEARCERAM-Z HS

We evaluated depth of subsurface damage on a ground surface of the ultra low expansion glass-ceramics CLEARCERAMR®-Z HS (CC-Z HS) by Ohara Inc., which is one of the candidates for material for segmented mirrors of the Thirty Meter Telescope. We made polishing spots of Magnetorheological Finishing on the ground surface of CC-Z HS and measured exposed subsurface damage features on the spot surface. We also studied on hydrofluoric acid etching of the CC-Z HS ground surface, which is expected to be an effective method to remove a subsurface damage layer compared with time-consuming polishing. We etched small ground surfaces of CC-Z HS and evaluated its uniformity.