I work down the street from this lab, and two of my good friends work in it, running the big polishing machines. Got the VIP tour last fall when a mirror was being spun and melted. It’s quite a place.
They use very good glass because stress is a killer for such big mirrors. The glass is so good, and the annealing so good, that they have successfully repaired several surface blemishes, bubbles and cracks in these mirrors. Also, glass polishes way better than any other material.
My father helped build a couple aluminum 60" mirrors in the late sixties for telescopes used for infrared photometry (no imaging needed), but they were eventually replaced with glass mirrors to gain versatility. The glass used also has the great quality of a nearly zero temperature coefficient of expansion (TCE), so it’s a lot easier to maintain nanometer-level dimensional stability through the diurnal temperature cycle.
The back is polished because the mirror blank has to be attached very firmly to the polishing machine turntable, so a flat back surface is needed.
