Electron microscopy studies of contaminated telescope mirror samples exposed at Paranal

Telescope mirrors typically consist of glass-ceramic substrates coated with a thin layer of aluminum or protected silver. Airborne contaminants on such surfaces can significantly degrade their reflectivity, IR emissivity and light scattering properties and cause damage such as pinholes. After exposing mirror samples near the Very Large Telescope (VLT) on Cerro Paranal, we investigated the contaminant formation at the microstructural level using electron microscopy. We show contaminant damage mechanisms on aluminum coated compared to protected silver mirror samples.