The optical design of the Southern African Large Telescope High Resolution Spectrograph: SALT HRS

SALT HRS is a fiber-fed cross-dispersed echelle spectrograph designed foro high resolution and high efficiency seeing spectroscopy on the Southern African Large Telescope. The spectrograph, which has a dual channel white pupil design, uses a single R4 echelle grating, a dichroic beam-splitter, and volume phase holographic gratings as cross-dispersers. The echelle grating has 41.6 grooves/mm and is illuminated with a 200 mm diameter beam. This allows R = 16,000 with a, 2.2 '' fiber and complete wavelength coverage from 370 nm to 890 nm. Resolving powers of R approximate to 37,000 and 67,000 are obtained using images slicers. The dichroic beam-splitter is used to demagnify wavelength coverage between two fully dioptric cameras. The white pupil transfer optics are used to demagnify the pupil to 111 mm which ensures that the camera dimensions are kept reasonable whilst also allowing the efficient use of VPH gratings. The spectrograph optics are enclosed inside a vacuum tank to ensure atmospheric pressure and temperature changes. The entire spectrograph is mechanically and thermally insulated Construction of SALT HRS began at Durham University's Centre for Advanced Instrumentation in August 2007 and is expected to be complete in 2009. The spectrograph optical design is largely based oil work completed at the University of Canterbury's Department of Physics and Astronomy.