The coating of large ground-based astronomical telescope mirrors is always a challenge, and the Large Synoptic Survey Telescope (LSST) contains some of the largest monolithic mirrors ever made for astronomy: it has a 3.4-m-diameter convex secondary mirror and an 8.4-m-diameter concave primary mirror. Bare aluminum coating deposited by evaporation is the most common type of coating deposited on such large mirrors. However, following the success of the Gemini Telescope coatings, LSST has chosen to develop a similar process based on magnetron sputtering deposition. In this paper, the specific requirements for coating the large LSST mirrors are presented, and the solutions developed to meet these specifications are described, including the design features and the major performance requirements.
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