Large telescope mirrors are typically coated with aluminum for reflectivity, but this aluminum can oxidize to form aluminum oxide, which is protective and transparent to visible light. To prevent rapid oxidation, these coatings are often overlaid with materials like silicon monoxide, enhancing reflectivity and extending the lifespan of the aluminum layer. Most reflector telescopes, including amateur models, utilize this aluminization technique. The protective coatings ensure that the mirrors maintain their performance over time. Overall, the use of aluminum and protective layers is standard practice in telescope mirror construction.
