"The casting of the secondary mirror blank for ESO's Extremely Large Telescope (ELT) has been completed by SCHOTT at Mainz, Germany. The completed mirror will be 4.2 metres in diameter and weigh 3.5 tonnes. It will be the largest secondary mirror ever employed on an optical telescope and also the largest convex mirror ever produced."
Dean
https://www.sciencedaily.com/release...0522080934.htm