Lessons learned from wet washing the 6.5m MMT primary mirror in-situ

Abstract

After gathering spectrophotometer data from the 6.5m primary mirror at the MMT Observatory (MMT) for over ten years, the MMT has developed a soap and water wash procedure that effectively removes contaminates from the surface of the mirror without damaging the bare aluminum coating. While the in-situ soap and water wash requires a small amount of telescope downtime, these washes are still scheduled to take place every six months. The frequency of the washing was selected to keep the mirror performance as close to a fresh coating as possible throughout the year and to extend the recoating interval without allowing the reflectivity degrading more than 3% from a fresh coating. After being in service for almost two years, the spectrophotometer measurements indicate the 2016 primary mirror coating is on-track to maintain the specified reflectance for five years. This paper outlines the soap and water wash procedure developed for the MMT primary mirror and presents spectrophotometer data throughout the life of the 2005 and the 2016 mirror coatings.