"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
Interestingly, the 4.2 metre secondary mirror is significantly bigger than the segments making up the primary (they're "only" 1.4 metres across - but there are a LOT of them - 798, to be precise). https://www.eso.org/public/teles-instr/elt/elt_num/
Amazing...4m was the standard primary mirror size not that long ago (70's and 80's) and while it's been 8-10m for the last 20 years or so, this scope is going to smash that standard ...cool times ahead