Kuzmenko, Paul J.; Little, Steve L.; Ikeda,
; Kobayashi, Naoto
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.