Gemini multiobject spectrograph optomechanical design

ABSTRACT The Gemini 8 - meter telescopes under construction on Mauna Kea, Hawaii and Cerro Pachon, Chile will each be equippedwith a high performance multi object spectrograph. Design and construction of the Gemini Multi Object Spectrograph(GMOS) is carried out by a team of scientists and engineers from both Canada and UK and involves the mounting of somerelatively large optical elements. The elements range in size from I 36mm to 300mm in diameter and are fabricated from avariety of glasses with quite different thermal and mechanical properties. Each GMOS unit will contain 10 different lensgroups with each group varying in complexity from 1 to 4 elements.A novel mounting scheme was devised for each group utilizing elastomers, subcells with matching coefficients of thermalexpansion (CTh), and optical couplants. The resulting groups achieve both radial and axial athermalization, in a relativelysimple mechanical configuration. The groups are also easily disassembled and reassembled (e.g. for coating the outersurfaces) without requiring realignment.The initial assembly actually corrects for any discrepancies between the optical axis and the mechanical axis of the lenses, andresults in a lens group with common optical axis. The group is contained in an outer aluminum structure that can easily bemounted I