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1 July 1990 Evaporative coating systems for very large astronomical mirrors
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Abstract
The design of aluminization systems for the MMT Conversion 6.5 m mirror and the Columbus Project 8 m mirror has led us to reconsider many of the design issues and tradeoffs for such systems. Coating of the large honeycomb mirrors will be done in situ on the telescope with a portable bell jar forming the front half of a two-stage vacuum system. The mirror cell forms a "dirty" vacuum behind the mirror to eliminate excess force on the glass. A multi-ring source geometry has been proposed to allow a 1.0 m spacing between the mirror surface and the sources thereby minimizing the size of the vacuum chamber. Evaporation source models have been developed to optimize the number of sources, ring spacing, and high incidence angle emission to achieve better than 5% rms deviation in coating thickness over the diameter. Code results are compared to empirical thickness profiles measured at the University of Arizona's (UA) Sunnyside 2.0 m coating facility. Cryoadsorption pumps are considered for reasons of economy, quality of vacuum, pumping speed, and reliability. The interaction of the cryopumps and getter pumping with the pumping/cleaning/deposition cycle is studied. Glow discharge cleaning is discussed and the results of deposition tests in 10' Torr residual argon are given. Electrical requirements are estimated and a novel transformer design may decrease the current entering the chamber from 12,000 A to less than 600 A.
© (1990) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
B. A. Sabol, Bruce Atwood, John M. Hill, Joseph T. Williams, Michael P. Lesser, Paul Laurence Byard, and Warren B. Davison "Evaporative coating systems for very large astronomical mirrors", Proc. SPIE 1236, Advanced Technology Optical Telescopes IV, (1 July 1990); doi: 10.1117/12.19262; https://doi.org/10.1117/12.19262
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