That's cool. But I am wondering how they keep that big a piece of glass from sagging. I am asking because I have read that's a problem with big lenses, and a key reason reflectors became the way to go for large telescopes.
It is a myth.
https://en.wikipedia.org/wiki/Glass#Behavior_of_antique_glas...
This problem is much simpler to solve for mirrors than lenses, as mirrors can be supported under their entire surface (see [0] for how it's done in larger amateur telescopes), while lenses - only along the edge.
[0] https://www.cloudynights.com/topic/562906-18-point-flotation...