VLT main mirror cleaning and recoating

Every night the huge mirrors of the Very Large Telescope are exposed to the atmosphere whilst uncovered during observing sessions. They gradually accumulate dust and other pollutants that reduce their reflectivity, making them less effective at capturing faint light from the cosmos. So they are regularly removed from the telescope, taken down the mountain to the recoating facility, cleaned and finally recoated with a thin and highly reflective new aluminium layer.

See more about the process on the ESOcast 120: Chile Chill 10: VLT Main Mirror Recoating .