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Have you ever wondered how a telescope keeps its mirrors in the best condition to observe the cosmos? In today’s Picture of the Week, a truck carefully carries one of the mirrors of ESO’s Very Large Telescope (VLT), wrapped to protect it from the harsh environment of Chile’s Atacama Desert. Its destination is the recoating facility that keeps the mirrors of this telescope perfectly shiny.
Despite being housed in enclosures that protect them from the extreme desert conditions during the day, telescope mirrors are still exposed at night, and therefore they need to be cleaned and recoated to keep their reflectivity. Dust that accumulates on the surface is regularly removed by spraying frozen carbon dioxide. Then, every 18 months or so, the mirror receives a new aluminium coating. For that, the mirror has to be removed from the telescope and slowly transported downhill to the recoating facility, a couple of kilometres away at basecamp. As the mirror is driven along, it is closely monitored by ESO staff walking alongside the transportation truck. ESO’s Extremely Large Telescope (ELT) — currently under construction — also watches silently from Cerro Armazones, seen in the background of this image.
At the recoating facility, the 8.2-metre mirror is separated from its supporting cell, a structure that protects the mirror and maintains its shape, and cleaned to remove contaminants that could damage the coating process. The thin aluminium layer, crucial for the mirror’s reflectivity, is removed with a chemical wash and replaced with a new one. After a process that takes about 8 days, including tests to verify the results, the restored mirror is then driven back up to the VLT, where it can get back to work, collecting light from deep space.