Development of advanced silver coatings for telescope mirrors

Several telescopes like VISTA or the ELT are using or will use silver coatings, replacing aluminum (Al). The advantage of silver is a higher overall reflectivity, in particular around 825 nm. Yet, silver must be protected by covering layer(s), which lowering the reflectivity in the blue/UV region. Therefore, ESO completed a 2-year development with Fraunhofer IOF under the objective of extending the wavelength range of high reflectivity to shorter wavelengths without decreasing the coating durability. The developed coatings have been examined by standardized tests like scratching sensitivity, peeling, salt mist and H2S gas. The quasi-standard in silver coatings for telescope mirrors was developed for the Gemini observatory and it uses nickel chromium nitride (NiCrNx) as interlayer between silver and the protective top layer(s), finished by a hard silicon nitride (Si3N4) layer of ∼ 15 nm thickness. We replaced the NiCrNx by aluminium oxide (AlOx) and it led to higher reflectivity but low durability towards the salt mist test. This test is relevant for many large telescopes because of their proximity to oceans, causing salty airborne dust. By replacing the NiCrNxby a ruthenium-based layer and by optimizing the protection, the objective of extending the wavelength range of high reflectivity to shorter wavelengths without decreasing the coating durability could be achieved.