Mechanical & thermal design challenges in building a semi-cold near infrared spectrograph. The Robert Stobie - Near Infrared Spectrograph for SALT

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 degrees 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 degrees 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 degrees 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.