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Silver vs. Aluminum

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Below are reflectance plots for coatings of bare aluminum (Al), bare silver (Ag), and silver with various protective layers. Gemini uses the 4-layer coating on all three telescope mirrors (primary, secondary, and science fold). The emissivity, E, of these coatings is related to the reflectance, R, by the equation E=1-R.

Protected silver has higher reflectance than aluminum at all wavelengths longer than ~450nm. Throughout the thermal infrared, where emission is an important source of contaminating background radiation, protected silver, which has emissivity of a little over 1%, offers a 2-3 times reduction in emissivity over aluminum. Below 450nm silver is inferior to aluminum; its reflectance drops to zero near 310nm.

The plots are from Boccas et al. 2006, Thin Solid Films, 502, 275

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