15 October 2003 Alignment of the grating wheel mechanism for a ground-based cryogenic near-infrared astronomy instrument
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Abstract
We describe the population, optomechanical alignment, and alignment verification of near-infrared gratings on the grating wheel mechanism (GWM) for the Infrared Multi-Object Spectrometer (IRMOS). IRMOS is a cryogenic (80 K), principle investigator-class instrument for the 2.1 m and Mayall 3.8 m telescopes at Kitt Peak National Observatory, and a MEMS spectrometer concept demonstrator for the James Webb Space Telescope. The GWM consists of 13 planar diffraction gratings and one flat imaging mirror (58x57 mm), each mounted at a unique compound angle on a 32 cm diameter gear. The mechanism is predominantly made of Al 6061. The grating substrates are stress relieved for enhanced cryogenic performance. The optical surfaces are replicated from off-the-shelf masters. The imaging mirror is diamond turned. The GWM spans a projected diameter of ~48 cm when assembled, utilizes several flexure designs to accommodate potential thermal gradients, and is controlled using custom software with an off-the-shelf controller. Under ambient conditions, each grating is aligned in six degrees of freedom relative to a coordinate system that is referenced to an optical alignment cube mounted at the center of the gear. The local tip/tilt (Rx/Ry) orientation of a given grating is measured using the zero-order return from an autocollimating theodolite. The other degrees of freedom are measured using a two-axis cathetometer and rotary table. Each grating's mount includes a one-piece shim located between the optic and the gear. The shim is machined to fine align each grating. We verify ambient alignment by comparing grating diffractive properties to model predictions.
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Sharon M. Gutkowski, Raymond G. Ohl, Jason E. Hylan, John G. Hagopian, Stephen E. Kraft, J. Eric Mentzell, Joseph A. Connelly, Joseph P. Schepis, Leroy M. Sparr, Matthew A. Greenhouse, and John W. MacKenty "Alignment of the grating wheel mechanism for a ground-based cryogenic near-infrared astronomy instrument", Proc. SPIE 5172, Cryogenic Optical Systems and Instruments X, (15 October 2003); doi: 10.1117/12.503506; https://doi.org/10.1117/12.503506
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