Cleaning the mirrors of giant telescopes is very complex
– News of September 23, 2018 –
Telescope mirrors are made with great care, but they end up being exposed to the elements during their observation period. Dust and other impurities can accumulate, reducing their performance. The mirrors must therefore be regularly maintained. They are often cleaned with dry ice. The carbon dioxide takes a solid form at minus 81 degrees, it is sent in this form to the surface of the mirrors. While warming up, it will quickly expanse and then resume a gaseous form, as a kind of mini-explosion that expels all dust and leaves no residue.
But even this regular maintenance is not enough. The mirrors must also pass several times during their life by a phase of realuminage. In this case, all the reflective coating of the mirror must be redone. The mirrors of the VLT (Very Large Telescope) undergo this maintenance every 18 months. For telescopes whose primary mirror is segmented like the ELT, the segments can be treated progressively. Only a few of them will undergo the procedure at the same time but as it is composed of 798 segments, it will almost be a daily process.
Image by ESO / L. Calçada / ACe Consortium