A tough fix

© Gerhard HudepohlIt is known that the Paranal Observatory has some of the biggest telescopes in the world, with its 8.2-meter mirrors. These huge mirrors need maintenance once in a while and the procedure to do this is far from easy, giving it credit enough to get coverage from National Geographic and be considered one of the toughest fixes in the world. These mirror coatings take typically a full week and are the most demanding periods from an engineering point of view, involving the coordination of all the departments and contractor companies. First of all, the mirror, together with its structure (cell), have to be removed from the telescope and transported 3 kms downhill to the coating building. This is one of the most critical moments, and the weather conditions may delay this operation for several days. Afterwards, the mirror alone is removed from the cell and put inside a clean room, where the coating procedure starts. The goal is to remove the aluminium layer from its surface, clean the glass mirror and replace the aluminium, aiming at the biggest reflectivity possible. The mirror is sprinkled with an acid-base solution to remove all the aluminium and then washed with demineralized water (see photo above). After assuring the cleanliness of the glass surface, the mirror is put into a vacuum chamber and coated with a thin aluminium layer. Finally, the mirror goes back to his cell and is brought up to the telescope again. The procedure is repeated for each of the four telescopes every 18 months approximately and while the mirror is being coated, other kinds of mechanical and electrical maintenance are performed inside the telescopes. Today, the mirror was put back to the telescope and another coating procedure is officially and successfully over. On previous coatings I had been helping with electronic maintenance inside the telescopes and this was the first time I was actually inside the clean room with the mirror. It is indeed impressive, to see a 8.2-m diameter and 20-cm thick piece of glass held defencelessly without its shell, another impressive thing about Paranal to sum to all the others.