The AAO's Gemini High-resolution Optical SpecTrograph (GHOST) concept

The Gemini High-Resolution Optical SpecTrograph (GHOST) will fill an important gap in the current suite of Gemini instruments. We will describe the Australian Astronomical Observatory (AAO)-led concept for GHOST, which consists of a multi-object, compact, high-efficiency, fixed-format, fiber-fed design. The spectrograph itself is a four-arm variant of the asymmetric white-pupil echelle Kiwispec spectrograph, Kiwisped, produced by Industrial Research Ltd. This spectrograph has an R4 grating and a 100mm pupil, and separate cross-disperser and camera optics for each of the four arms, carefully optimized for their respective wavelength ranges. We feed this spectrograph with a miniature lenslet-based IFU that sub-samples the seeing disk of a single object into 7 hexagonal sub-images, reformatting this into a slit with a second set of double microlenses at the spectrograph entrance with relatively little loss due to focal-ratio degradation. This reformatting enables high spectral resolution from a compact design that fits well within the relatively tight GHOST budget. We will describe our baseline 2-object R∼50,000 design with full wavelength coverage from the ultraviolet to the silicon cutoff, as well as the high-resolution single-object R∼75,000 mode.