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Progress in the fabrication of a prototype ZnSe immersion grating for the WINERED spectrograph

Kuzmenko, Paul J.; Little, Steve L.; Ikeda, Yuji; Kobayashi, Naoto
2010, SPIE
■ アブストラクト
The WINERED spectrograph requires a large (9 cm × 9 cm aperture), 70° blaze ZnSe immersion grating to achieve a resolution of ~100,000. It will be implemented as a mosaic of 3 gratings of aperture 3 cm × 9 cm. LLNL is diamond machining a subscale prototype (2.3 cm × 5.0 cm aperture) to demonstrate feasibility. This is a 10× increase in the size of gratings machined at LLNL. The first cutting of the grating had large wave front errors due to non-working thermal control. After repairs we recut a grating with an rms wavefront error of 0.11 wave @633 nm, which meets WINERED's full aperture requirement. The first cutting had good quality grooves, but the recut showed bad chipping. A second recut after polishing away the grating had pitted groove faces. It is known that diamond machining and mechanical polishing create subsurface damage. This damage can cause brittle fracture during subsequent machining. We plan to chemically etch ZnSe substrates to remove damaged layers prior to machining our next gratings.