Project status of the Robert Stobie spectrograph near infrared instrument (RSS-NIR) for SALT

Abstract

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.