Long-term reflectivity and scattering data of MMT primary mirror (M1) coating from 2016 to the present suggest minimal optical degradation, resulting, in part, from periodic wet cleaning of the coating. This extended (approximately four-year) period of coating maintenance and accompanying optical sampling provides a more complete picture of M1 coating performance and of the contribution of periodic cleaning to that performance. Semi-annual, soap-and-water cleanings have helped maintain the coating’s optical quality. Techniques for cotton swab cleaning and for optical sampling of the aluminum coating with a Konica-Minolta CM-600d Spectrophotometer are discussed. These results imply that periodic, well-controlled cleaning has significantly extended the useful life of the 2016 MMT primary mirror coating. With the goal of maintaining a five to six year coating cycle.
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