Largest convex mirror

In May 2017, German company Schott cast the secondary mirror (M2) for the Extremely Large Telescope (ELT) that is due to be finished by the European Southern Observatory (ESO) in 2024. At 4.2 metres (13 ft 9.35 in) this extraordinary mirror is made from a type of glass ceramic called Zerodur®. This material has extremely low thermal expansion, making it ideal for telescopes where image quality is everything. At 3.5 tonnes (7,716 lb), this secondary mirror of the telescope took a whole year to cool down (it was also heated and cooled slightly over this time to remove all the stress in the material that could break it). Now it is cooled and shaped, the mirror will be highly polished and coated ready for when the rest of the ELT is ready. It is the largest convex mirror ever made to date, and also the largest secondary mirror of any optical telescope.