Progress in the development of a durable silver-based high-reflectance coating for astronomical telescopes

Silver-based high-reflectance coatings that can withstand the humid and polluted conditions common in the open air have been developed for astronomical telescope optics. The successful designs incorporate a silver reflective layer with a copper underlayer and a stack of dielectric overlayers. Prototypes have been deposited and tested in a controlled environmental chamber and in true operating conditions on Kitt Peak in Arizona. The improved durability, which is due to the copper underlayer, has been investigated with analytical techniques, including Rutherford backscattering.