Dust, dirt, cinder, and other particulates on our telescopes’ primary and secondary mirror surfaces can significantly degrade their reflectivity and light scattering properties. To keep our 8.2-m primary mirrors and 1.0-m secondary mirrors shiny and reflective between our recoating cycles (every few years we re-silver the top coat of our mirrors), we clean the mirror as part of our weekly telescope maintenance.
This might be different from what you’re expecting. We’re not using soap and water or isopropyl alcohol. We use carbon dioxide snow or dry ice instead. The telescope is tipped over close to the horizon so the mirrors are nearly perpendicular to the ground. Using wands hooked up to liquid carbon dioxide kept under pressure, the day crew sprays the mirror. Once the liquid carbon dioxide leaves the nozzle it nearly instantly expands to become solid ice crystals. The these carbon dioxide ice crystals are typically on the order of the nozzle diameter (typically a few mm or smaller).
The crystals quickly sublimate (changing from solid to gas) when they hit the surface of the mirror. This impact and expansion blow off the dust and particles off the mirror’s surface. Particulates on the mirror surface are carried away floating on top of the layer of gas created by the sublimating carbon dioxide ice particles. This process in essence scrubs the mirror but is gentle on the very thin silver coating we have on our mirror. They simply fall off, which is why w
Here’s a video captured by Science Operations Specialist Christy Cunningham showing the Gemini Maunakea day crew cleaning the Gemini North mirror and spraying the carbon dioxide snow onto the 8.2-m primary mirror.
You can see how our next door neighbors at the Subaru Telescope perform their carbon dioxide snow mirror cleaning here.