The Giant Magellan Telescope (GMT) is a 25.4-m diameter, optical/infrared telescope that is being built by an international consortium of universities and research institutions as one of the next generation of Extremely Large Telescopes. The primary mirror of GMT consists of seven 8.4 m borosilicate honeycomb mirror segments that are optically conjugate to seven corresponding segments in the Gregorian secondary mirror. Fabrication is complete for one primary mirror segment and is underway for the next two. The final focal ratio of the telescope is f/8.2, so that the focal plane has an image scale of 1.0 arcsec/mm. GMT will be commissioned using a fast-steering secondary mirror assembly comprised of conventional, rigid segments to provide seeing-limited observations. A secondary mirror with fully adaptive segments will be used in standard operation to additionally enable ground-layer and diffraction-limited adaptive optics. In the seeing limited mode, GMT will provide a 10 arcmin field of view without field correction. A 20 arcmin field of view will be obtained using a wide-field corrector and atmospheric dispersion compensator. The project has recently completed a series of sub-system and system-level preliminary design reviews and is currently preparing to move into the construction phase. This paper summarizes the technical development of the GMT sub-systems and the current status of the GMT project.
Acting as a prime contractor to the Gemini PrOJeCt [GPJ, the Optical Data Associates [ODA] . with Its major subcontractors, BOC Coating Technologies IBOCC11 and Deposition Sciences, Inc. IDSII, developed options for depositing protected silver coatings on the 8-M primary mirroi. The project began with a study that identified sputtering as the preferred deposition technique, defined a set of candidate adhesor and protective coatings for the silver layer, and simulated stack performance. The next phase involved pilot magnetron sputtering studies by BOCC and DSI of designs lnvoMng silicon nitride and hafnla, respectively. ODA also developed mid-IR reflectance standards at . = 10.6 pm to control the silver coating measurements In the critical 8-12 pin atmospheric window. The study. results were successful, with both BOCCI and DSI producing Ag coatings with R = 0.9920 0.0001 and protected Ag coatings with R = 0.9910 0.0001 . The Gemini Project coating plants are designed to sputter bare and protected Al and Ag coatings.
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