Production of the world’s largest convex ZERODUR mirror blank for the ELT

In January 2019 SCHOTT has successfully delivered the 4.25 m diameter convex ZERODUR® mirror blank for the secondary mirror (M2) of the extremely large telescope (ELT) of ESO, followed by 4 m ZERODUR® mirror blank for the tertiary mirror (M3) in December 2019. The ELT will be the largest telescope in the world with a primary mirror of 39 m aperture. Additionally to the M2 and M3 blanks all 798 segments of primary mirror M1 and the thin shells for the world's largest adaptive M4 mirror are made of ZERODUR®. With its 4.25 m physical diameter, the ELT M2 mirror will be the largest convex secondary mirror in the world. The production for the M2 and M3 started with the melting and casting of 4.5 m diameter raw blank. The coefficient of thermal expansion was optimized during ceramization to ZERODUR® TAILORED grade for the application temperature conditions of the ELT on mount Antofagasta in Chile. The installation of a new high accuracy 5 m size CNC grinding center enabled an excellent surface shape accuracy on the functional surface. The sub-surface damage of the back surface and the outer diameter was reduced by means of an acid etching process to increase the lifetime of the bonding interfaces to the support structure. Dedicated handling equipment was designed and build in-house to support the production process. This presentation gives an overview on the tasks and the achieved results of the ELT M2 and M3 ZERODUR® blanks.