Cleaning a VLT mirror

The 8.2m diameter main mirror of Antu, the first Unit Telescope of ESO's Very Large Telescope, is being cleaned using carbon dioxide snow.

While the telescope enclosure is maintained extremely clean, the mirrors are exposed to the elements during the observations. Consequently, dust from the desert slowly accumulates over the surface of the mirror, making it less reflective over time.

The mirror's surface is so delicate that normal cleaners used for household mirrors are not appropriate for telescopes. Observatories have developed other methods, such as this one using carbon dioxide snow. The tiny CO2 snowflakes in the white plume have a temperature of almost minus 80 degrees Celsius; when they land on the mirror, which is at room temperature, they cause minuscule 'explosions' that detach the dust grains from the surface. The dust then floats away, leaving the mirror clean. The process is nevertheless very delicate: should Alain Gilliotte, the optician performing the cleaning, let the CO2 device touch the mirror, the fragile reflective Aluminium coating would be scratched. Also, hair or cloth lint should stay away from the mirror, which is why the optician is wearing a white suit made of special plastic.