NTT mirror being washed

The main mirror of the NTT is removed from the telescope about twice a year for cleaning. Indeed, dust accumulates over time on its surface, absorbing and diffusing some of the light that should be focused on the instrument. Cleaning this 3.56m diameter mirror is a delicate task, but the concept is simple : very soft natural sponges are used together with a very soft soap, and very clean water. The 3.6m telescope is seen in the background, and by reflection on the NTT mirror being washed.