The precious metal used to coat the mirrors of the James Webb Space Telescope was gold.
Webb’s primary mirror has 18 hexagonal segments, while its secondary, tertiary, and fine-steering mirrors complete the telescope’s optical system. The mirror surfaces themselves are made from lightweight beryllium, chosen for its stiffness, low density, and stability in the extremely cold environment of space.
Gold was selected because it reflects infrared light exceptionally well—the wavelengths Webb is designed to observe. Aluminum and silver can also be used as mirror coatings, but gold offered better performance across Webb’s infrared range and greater durability for the mission. The coating is extraordinarily thin, about 100 nanometers, or roughly one-thousandth the thickness of a sheet of paper.
Despite the telescope’s golden appearance, only about 50 grams of gold coat its mirrors. The gold was applied by vacuum vapor deposition and covered with a thin protective layer of silicon dioxide to help guard the soft metal against scratches and contamination.