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9 August 2016 Polishing test of a poly-crystal calcium fluoride lens: toward the development of TMT WFOS
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Wide-Field Optical Spectrograph (WFOS) is one of the first-light instruments of Thirty Meter Telescope (TMT), and developed in an international collaboration led by University of California Santa Cruz. It covers the wavelength range from 310 nm to 1 μm which is divided at around 550 nm by a dichroic mirror. Calcium Fluoride (CaF2) is very useful to reduce aberration and has good transmittance even at 310 nm. Because a large mono-crystal CaF2 is difficult to be manufactured, we might have to use a poly-crystal CaF2. Comparing a mono-crystal, the poly-crystal is expected to have worse optical index homogeneity and larger surface figure error after polishing. Those effects on an image quality are unclear. To verify those effects, we conducted a polishing test of a small poly-crystal CaF2 lens as a first step. As a result, we found figure error around the boundary. The figure error is ~139 nm PV and ~26 nm RMS. Comparing a Zemax simulation, it is confirmed that the figure error does not have significant effect on the image quality.
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Shinobu Ozaki, Satoshi Miyazaki, Toshihiro Tsuzuki, and Yoko Tanaka "Polishing test of a poly-crystal calcium fluoride lens: toward the development of TMT WFOS", Proc. SPIE 9908, Ground-based and Airborne Instrumentation for Astronomy VI, 99089N (9 August 2016);

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