Designing the optimal semi-warm NIR spectrograph for SALT via detailed thermal analysis

Marsha J. Wolf; Andrew I. Sheinis; Mark P. Mulligan; Jeffrey P. Wong; Allen Rogers

doi:10.1117/12.788066

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9 July 2008 Designing the optimal semi-warm NIR spectrograph for SALT via detailed thermal analysis

Marsha J. Wolf, Andrew I. Sheinis, Mark P. Mulligan, Jeffrey P. Wong, Allen Rogers

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Proceedings Volume 7014, Ground-based and Airborne Instrumentation for Astronomy II; 701432 (2008) https://doi.org/10.1117/12.788066
Event: SPIE Astronomical Telescopes + Instrumentation, 2008, Marseille, France

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.

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Marsha J. Wolf, Andrew I. Sheinis, Mark P. Mulligan, Jeffrey P. Wong, and Allen Rogers "Designing the optimal semi-warm NIR spectrograph for SALT via detailed thermal analysis", Proc. SPIE 7014, Ground-based and Airborne Instrumentation for Astronomy II, 701432 (9 July 2008); https://doi.org/10.1117/12.788066

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