Protected-silver coatings for the 8-m Gemini telescope mirrors

The twin 8-m diameter Gemini telescopes were designed to use silver-based coatings on the mirrors in order to provide very high reflectivity and ultra-low emissivity for optimal infrared performance. A feasibility study provided both techniques and recipes to apply these thin films, and showed that a reflectivity of 99.1% at 10 μm was achievable. We have now produced bare and protected silver sputtered films in our coating plants and conducted environmental testing, both accelerated and in real-life conditions, to assess the durability in an observatory environment. We have also already applied, for the first time ever, protected-silver coatings on the main optical elements of a large telescope. We report here the performance of the films, the challenges to coat a 50 m² primary mirror (M1) and our plans for coating maintenance.