Come on. Petapixel is a blogspam site. At the bottom of the posts you can find the source. In this case it's "via Engadget":

https://www.engadget.com/2019/09/23/moving-the-largest-high-...

Also helps when you need to burn some very large ants.

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.

Because glass does not sagg.

It is a myth.

https://en.wikipedia.org/wiki/Glass#Behavior_of_antique_glas...

You are mistaking two effects. Indeed, glass does not "flow" (it's not a "very, very viscous liquid"); but most assuredly it does sag under gravity - just as a heavy steel beam supported at the ends would.

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...

Total conjecture but it was probably factored in. From my standpoint if I knew it was going to sag, I would use that to my advantage if I could and, if not, at least factor it in.

If you factor it in, it means you can't reorientent the lens, since it relies on gravity to have the right shape.

spin it occasionally

see gattr reply above.

Article misses the big question: why did they put all this work into developing a solid lens rather than a mirror or a Fresnel lens?

My guess is a mirror wouldn't fit into the design, and a Fresnel lens would not have the needed accuracy for astronomical work.