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I mean... the obvious answer to that is that we don't use the same techniques to manufacture a telescope or camera lens that we used to manufacture church windo

I mean... the obvious answer to that is that we don't use the same techniques to manufacture a telescope or camera lens that we used to manufacture church windows. In fact we don't use glass for large telescopes specifically because they'll deform under their own weight. I'm not saying that original fact is correct, I'm just saying I don't think what you're saying disproves the idea that that glass could deform over time.

gattr 6y ago
That's incorrect. We do use glass for large mirrors and it works just fine ([1], [2.1], [2.2]). Do not confuse these two effects: 1. Viscous liquid-like creep (does not occur). 2. Elastic deformation (sagging) under gravity - this of course takes place, and is compensated for by multipoint support and, in larger professional telescopes, "active optics" (actuactors that counter the deformation depending on mirror's orientation; not to be confused with "adaptive optics", which deform a mirror with high frequency to counteract atmospheric image distortion). [1] https://en.wikipedia.org/wiki/Hale_Telescope https://en.wikipedia.org/wiki/Hale_Telescope [2.1] https://en.wikipedia.org/wiki/Large_Binocular_Telescope https://en.wikipedia.org/wiki/Large_Binocular_Telescope [2.2] https://www.newswise.com/articles/steward-observatory-mirror-lab-to-cast-2nd-84-meter-lbt-mirror https://www.newswise.com/articles/steward-observatory-mirror...