KiwiSpec - an advanced spectrograph for high resolution spectroscopy: Optical design and variations

The Kiwi Spec R4-100 is an advanced high resolution spectrograph developed by Kiwi Star Optics, Industrial Research Ltd, New Zealand. The instrument is based around an R4 echelle grating and a 100 mm collimated beam diameter. The optical design employs a highly asymmetric white pupil design, whereby the transfer collimator has a focal length only 1/3 that of the primary collimator. This allows the cross-dispersers (VPH gratings) and camera optics to be small and low cost while also ensuring a very compact instrument. The Kiwi Spec instrument will be fibre-fed and is designed to be contained in both thermal and/or vacuum enclosures. The instrument concept is highly flexible in order to ensure that the same basic design can be used for a wide variety of science cases. Options include the possibility of splitting the wavelength coverage into 2 to 4 separate channels allowing each channel to be highly optimized for maximum efficiency. CCDs ranging from smaller than 2K x 2K to larger than 4K x 4K can be accommodated. This allows good (3-4 pixel) sampling of resolving powers ranging from below 50,000 to greater than 100,000. Among the specific design options presented here will be a two-channel concept optimized for precision radial velocities, and a four-channel concept developed for the Gemini High-Resolution Optical Spectrograph (GHOST). The design and performance of a single-channel prototype will be presented elsewhere in these proceedings.(1)