NTT mirror

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. On this photo, optician Alain Gilliotte is measuring the reflectivity of the mirror before the cleaning operation starts.