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Proceedings Paper

Designing the optimal semi-warm NIR spectrograph for SALT via detailed thermal analysis
Author(s): Marsha J. Wolf; Andrew I. Sheinis; Mark P. Mulligan; Jeffrey P. Wong; Allen Rogers
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Paper Abstract

The near infrared (NIR) upgrade to the Robert Stobie Spectrograph (RSS) on the Southern African Large Telescope (SALT), RSS/NIR, extends the spectral coverage of all modes of the optical spectrograph. The RSS/NIR is a low to medium resolution spectrograph with broadband, spectropolarimetric, and Fabry-Perot imaging capabilities. The optical and NIR arms can be used simultaneously to extend spectral coverage from 3200 Å to approximately 1.6 μm. Both arms utilize high efficiency volume phase holographic gratings via articulating gratings and cameras. The NIR camera incorporates a HAWAII-2RG detector with an Epps optical design consisting of 6 spherical elements and providing subpixel rms image sizes of 7.5 ± 1.0 μm over all wavelengths and field angles. The NIR spectrograph is semi-warm, sharing a common slit plane and partial collimator with the optical arm. A pre-dewar, cooled to below ambient temperature, houses the final NIR collimator optic, the grating/Fabry-Perot etalon, the polarizing beam splitter, and the first three camera optics. The last three camera elements, blocking filters, and detector are housed in a cryogenically cooled dewar. The semi-warm design concept has long been proposed as an economical way to extend optical instruments into the NIR, however, success has been very limited. A major portion of our design effort entails a detailed thermal analysis using non-sequential ray tracing to interactively guide the mechanical design and determine a truly realizable long wavelength cutoff over which astronomical observations will be sky-limited. In this paper we describe our thermal analysis, design concepts for the staged cooling scheme, and results to be incorporated into the overall mechanical design and baffling.

Paper Details

Date Published: 9 July 2008
PDF: 11 pages
Proc. SPIE 7014, Ground-based and Airborne Instrumentation for Astronomy II, 701432 (9 July 2008); doi: 10.1117/12.788066
Show Author Affiliations
Marsha J. Wolf, Univ. of Wisconsin, Madison (United States)
Andrew I. Sheinis, Univ. of Wisconsin, Madison (United States)
Mark P. Mulligan, Space Science and Engineering Ctr. (United States)
Jeffrey P. Wong, Paradigm Design Inc. (United States)
Allen Rogers, IceCube Neutrino Observatory (United States)

Published in SPIE Proceedings Vol. 7014:
Ground-based and Airborne Instrumentation for Astronomy II
Ian S. McLean; Mark M. Casali, Editor(s)

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