Evaporative Coating Systems for Very Large Astronomical Mirrors

B. A. Sabol, B. Atwood, J. M. Hill, J. T. Williams,
M. P. Lesser, P. L. Byard, W. Davison

MMT Observatory, University of Arizona
Tucson, AZ 85721

The Ohio State University, Department of Astronomy
5040 Smith Lab, Columbus, OH 43210-1106

Steward Observatory, University of Arizona
Tucson, ZA 85721

http://medusa.as.arizona.edu/lbtwww/tech/evapor.htm

Proceedings of SPIE conference on Advanced Technology Optical Telescopes IV, 1236, p. 940 (1990)

Abstract

The design of aluminization systems for the MMT Conversion 6.5 meter mirror and the Columbus Project 8 meter mirrors has led us to reconsider many of the design issues and tradeoffs for such systems. A multi-ring source geometry has been proposed to allow a 1 meter 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 and ring spacing to achieve better than 5% rms deviation in coating thickness over the diameter of the F/1.2 mirrors. A source aperturing option is included to maintain high reflectivity by suppressing oblique angle deposition. Code results are compared to empirical thickness profiles measured at the Steward Observatory 2.0 meter coating facility. Cryoadsorption pumps are considered for reasons of economy, quality of vacuum, pumping speed and reliability. The interaction of the cryopumps with the pumping - cleaning - deposition cycle has been designed to achieve an optimum aluminum coating. Results of deposition tests in 10-4 Torr argon are discussed. 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. Aluminizing on the telescope should minimize the risk associated with moving the mirror and/or its cell to an external coating facility. The design of a novel filament support structure, which incorporates a step-down transformer, is also presented. This scheme reduces the current which must be carried through the vacuum vessel and distributed among the approximately 250 sources from 12,000 amps to less than 600 amps.