VLT mirror ready to be washed

One of the VLT´s 8.2-metre primary mirrors, already removed from its supporting cell, is about to be put in a special washing machine where its thin reflective aluminium layer will be removed by chemicals. This is the riskiest moment of the whole recoating process, because the large mirror, weighing 23 tonnes and only 17 cm thick is hanging from the 15 hooks of this special crane (12 hooks around the perimeter and three at the central hole). While in its cell, the primary mirror is protected by sophisticated anti-seismic devices but during this operation — intended to be as fast as possible — it is much more vulnerable. Here an engineer is controlling the movement of the washing chamber which will be positioned under the crane, in order to put the mirror inside its dish. The metal structures visible on the bottom of the mirror are the tops for the actuators of the active optics system. Since telescopes spend the night under the stars, the mirrors accumulate on their surfaces a certain amount of dirt which reduces the reflectivity and degrades the image. The VLT 8.2-metre primary mirrors are therefore recoated every 18 months.