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Application of zero-expansion pore-free ceramics to a mirror of an astronomical telescope

We developed an aspheric convex 33-cm diameter secondary mirror of the Hiroshima University 1.5-m Ritchy-Chretien telescope using Zero-expansion Pore-free ceramics, which has physical properties (thermal expansion, stiffness, thermal conductance, etc.) comparable with or better than existing zero-expansion glasses. After high-precision grinding, polishing, and coating aluminum and silicon monoxide, we obtained the sufficient optical reflecting surface with a figure error within λ/10 and a roughness of about 3 nm rms. The mirror has been attached on the telescope and we confirmed its sufficient performance through a Hartmann test. To date it has shown a good performance in our application and we suggest that this ceramic material has a potential to be used for astronomical telescopes and related area.