The near infrared upgrade to the Robert Stobie Spectrograph (RSS/NIR) for the Southern African Large Telescope
(SALT) extends the capabilities of the visible arm RSS into the Near Infrared (NIR). In order to extend into the NIR
range, the upgrade components of the instrument are required to be cooled. Thus the NIR arm is predominantly housed
in the instrument pre-dewar which is cooled to -40°C, at ambient pressure. The multiple modes, prime focus location and
partially cooled instrument introduce interesting engineering considerations. The NIR spectrograph has an ambient
temperature collimator, a cooled (-40°C) dispersers and camera and a cryogenic detector. The cryogenic dewar and
many of the mechanisms are required to operate within the cooled, atmospheric environment. Cooling the pre-dewar to -
40°C at prime focus of the telescope is also an engineering challenge. Mechanical and thermal aspects of the design are
addressed in this paper with a particular emphasis on the unique considerations of building a semi-warm infrared
spectrograph.
The Robert Stobie Spectrograph Near Infrared Instrument (RSS-NIR), a prime focus facility instrument for the 11-meter
Southern African Large Telescope (SALT), is well into its laboratory integration and testing phase. RSS-NIR will
initially provide imaging and single or multi-object medium resolution spectroscopy in an 8 arcmin field of view at
wavelengths of 0.9 - 1.7 μm. Future modes, including tunable Fabry-Perot spectral imaging and polarimetry, have been
designed in and can be easily added later. RSS-NIR will mate to the existing visible wavelength RSS-VIS via a dichroic
beamsplitter, allowing simultaneous operation of the two instruments in all modes. Multi-object spectroscopy covering a
wavelength range of 0.32 - 1.7 μm on 10-meter class telescopes is a rare capability and once all the existing VIS modes
are incorporated into the NIR, the combined RSS will provide observational modes that are completely unique.
The VIS and NIR instruments share a common telescope focal plane, and slit mask for spectroscopic modes, and
collimator optics that operate at ambient observatory temperature. Beyond the dichroic beamsplitter, RSS-NIR is
enclosed in a pre-dewar box operating at -40 °C, and within that is a cryogenic dewar operating at 120 K housing the
detector and final camera optics and filters. This semi-warm configuration with compartments at multiple operating
temperatures poses a number of design and implementation challenges. In this paper we present overviews of the RSSNIR
instrument design and solutions to design challenges, measured performance of optical components, detector
system optimization results, and an update on the overall project status.
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