A UV-enhanced protected silver coating for the Gemini telescopes

A 4-layer protected silver coating was chosen for the Gemini telescopes due to its high reflectivity in visible and infrared wavelengths. Due to the reflective and absorptive properties of silver, the Gemini 4-layer coating suffers from poor reflectivity in wavelengths shorter than 400 nm. By utilizing different dielectric materials for protective layers it is possible to enhance the reflectance of silver to 350 nm, without compromising infrared performance or coating durability. Several different recipes were found that achieve a goal reflectivity of 80% at 350 nm and 90% at 400 nm. Five recipes were chosen for testing based on their modeled UV enhancement, the protective properties of their materials, and their simplicity. These recipes consist of variations of aluminum oxide, as well as silicon dioxide or silicon nitride. The aluminum oxide replaces the nickel chromium interlayer in the current Gemini coating and provides a receptive surface for the other protective layers. These materials were chosen because they can be produced in the Gemini coating facility without requiring significant and costly upgrades of the sputtering cathodes.