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The southern Gemini 8m Telescope on Cerro Pachon will be instrumented, as part of the high priority, first-light contingent of instruments, with a high resolution optical spectrograph (HROS). The Gemini HROS Instrument Working Group, comprised of J. Landstreet, M. Pettini, C. Pilachowski (chair), D. Walker, and D. York, was charged with developing the scientific and technical requirements for the spectrograph. The Working Group has recommended two regimes of spectral resolution as critical to Gemini science. A resolving power near 50,000, with optimization for high throughput, is essential for observations of faint sources such as QSO's, stars in other galaxies, and much stellar abundance work. A resolving power of 120,000, with primary emphasis on spectral accuracy and precision and secondary emphasis on throughput, is essential for work requiring high velocity resolution and line profile studies.
The outstanding image quality which the Gemini telescopes are designed to deliver and the excellent seeing characteristics of the Cerro Pachon site allow the physical dimensions of the instrument to be reduced compared to spectrographs designed for other large telescopes, while still maintaining high throughput. The spectrograph will be designed for a slit width of 0.6 arc seconds at a resolving power of 50,000 and 0.24 arc seconds at resolving power of 120,000. The HROS will be mounted on the Cassegrain focus of the southern 8m telescope, with an alternate fiber feed to the instrument in a thermally stable spectrograph laboratory in the telescope pier. The Cassegrain mounting drives a requirement for a high level of stability of the spectrum on the detector, of order less than two microns per hour. Such a requirement can probably be met with careful attention to mechanical design, thermal control, and open-loop correction of the optical alignment within the spectrograph. The scientific and technical requirements of the HROS and the recommendations of the Working Group will be presented.
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