Optics for a volume holographic grating spectrograph for the Southern Astrophysical Research (SOAR) Telescope

We describe a novel optical design for a low-resolution imaging spectrograph that incorporates volume phase holographic (VPH) gratings. This spectrograph will provide imaging over a 5' square field of view, and single or multi-object spectroscopy with resolutions between 1000 and 8000. Our design choices have been dominated by a desire to preserve the superb image quality the SOAR telescope is expected to deliver, and to maximize throughput over the wavelength range of 320 to 850 nm. The resulting design is unusual in two respects: the angle between the collimated beam and camera optical axis is mechanically variable to exploit the efficiency advantages of volume holographic gratings, and all the optical elements are refracting, to maintain high throughput in the ultraviolet. In addition to the pre-construction collimator and camera designs for the spectrograph, we also present our evaluation of sample volume phase holographic gratings to illustrate the advantages and difficulties they present for astronomical spectroscopy.