[fastball]

Why is the substrate glass? Lack of reactivity? Ability to remove imperfections? As a layman with almost zero knowledge of telescope construction, I feel like a heavy amorphous solid would not be my first choice for the base layer underneath the reflective/mirror coating.

[minetest2048]

Its not your normal soda-lime glass, its more of a glass-ceramic material that have very low coefficient of thermal expansion, something like zerodur: https://en.m.wikipedia.org/wiki/Zerodur , which means it can keep its shape and focus even under varying temperature

Interesting demo by Huygens Optics: https://youtu.be/qi8jmEbWsxU?si=rj0I3k-l74Xhg7vC

[perihelions]

I think while the smaller mirror here is Zerodur, the larger one is just borosilicate,

https://astro.arizona.edu/news/rubin-observatory-achieves-an...

[IAmBroom]

That would be an odd choice.

A large glass mirror once cracked because they "only" allowed one year for it to cool.

The second one was allotted three years.