GMOS: The GEMINI multiple object spectrographs

The two GEMINI Multiple Object Spectrographs (GMOS) are being designed and built for use with the GEMINI telescopes on Mauna Kea and Cerro Pachon starting in 1999 and 2000 respectively. They have four operating modes : imaging, long slit spectroscopy, aperture plate multiple object spectroscopy and area (or integral field) spectroscopy. The spectrograph uses refracting optics for both the collimator and camera and uses grating dispersion. The image quality delivered to the spectrograph is anticipated to be excellent and the design is driven by the need to retain this acuity over a large wavelength range and the full 5.5 arcminute field of view. The spectrograph optics are required to perform from 0.36 to 1.8 microns although it is likely that the northern and southern versions of GMOS will use coatings optimised for the red and blue respectively. A stringent flexure specification is imposed by the scientific requirement to measure velocities to high precision (1-2 km/s). Here we present an overview of the design concentrating on the optical and mechanical aspects.