Gemini primary mirror in situ wash

The Gemini twins were the first large modern telescopes to receive protected silver coatings on their mirrors in 2004. The low emissivity requirement is fundamental for the IR optimization. In the mid-IR a factor of two reduction in telescope emissivity is equivalent to increasing the collecting area by the same factor. Our emissivity maintenance requirement is very stringent: 0.5% maximum degradation during operations, at any single wavelength beyond 2.2 μm. We developed a very rigorous standard to wash the primary mirrors in the telescope without science down time. The in-situ washes are made regularly, and the reflectivity and emissivity gains are significant. The coating lifetime has been extended far more than our original expectations. In this report we describe the in-situ process and hardware, explain our maintenance plan, and show results of the coating performance over time.