*Ramya Chandrasekaran1, Carl Smith2, Robert A Pollara Jr.1, Thomas Bruce1, Thomas Foley1
1Dynavac, Hingham, MA; 2The Boeing Company, Albuquerque, NM
Reflective coatings on large astronomical mirrors have been predominately done using aluminum evaporation techniques, but the high reflectivity and low emissivity properties of silver makes this an attractive material choice for demanding visible and infrared mirror applications. Unfortunately, multilayer silver deposition using magnetron sputtering is a newer concept in large mirror applications involving increased technical and financial risk. Recently a vacuum system that was used to coat a 3.5 m telescope mirror using Aluminum evaporation was modified to sputter coat a multilayer silver stack. In this paper, we present the hardware configuration required to effectively sputter coat a multilayer silver-based coating. Critical design and process challenges will be presented and solutions will be described. Different overcoat layers have been developed to achieve high reflectivity and to protect silver mirrors from tarnish and corrosion. The effects of such layers were understood and optimized based on optical properties and environmental durability testing. Temperature measurements were performed on a simulated mirror section. The 3.5 m mirror was successfully coated with the silver-based coating in June 2019.