A telescope mirror in a state-of-the-art observatory needs to be a lot of things. But it also means that the telescope mirrors have to withstand temperature variations, without getting deformed. Optical telescope mirrors need to be super smooth because they observe short wavelengths, such as visible light. The mirror coating has a limited life span, as silver and aluminium slowly become oxidised when they are exposed to the air. So, how do we make sure that the 798 mirror segments of the ELT remain smooth and reflective?