The Giant Segmented Mirror Telescope (also known as the Thirty Meter Telescope) will consist of about 600 1.5 meter segments, each with an advanced silver based reflective coating. The mirror coatings must have high reflectance from the UV to far IR, with low emissivity near 0.01 in the NIR. This paper will describe design and deposition of advanced silver based multilayer coatings, and issues with achieving high reflectance over the broad wavelength range. In particular, the surface plasmon absorption at UV wavelengths, which degrades UV reflectance, has been virtually eliminated. Environmental and chemical degradation of the coating has also been eliminated.
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