3 ms·

Yeah, I learned this "fact" in school as well. Later I found a simple refutation: If a church window pane would deform by a measurable amount in 1000 years (say

Yeah, I learned this "fact" in school as well. Later I found a simple refutation: If a church window pane would deform by a measurable amount in 1000 years (say 0.5mm), then a 100-year old telescope or camera lens would be totally useless, and yet they seem to function all right.

gattr 6y ago
To add some detail: a common minimum quality criterion for optical surfaces is λ/4 (divergence from an ideal surface), where λ is the wavelength used. For visible light this gives about 100 nm (though large professional telescopes are "figured" to more like λ/20). Any alleged glass creep visible in window panes over a few hundred years would quickly ruin any astronomical mirror (which definitely does not happen).
Traster 6y ago
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...

gattr 6y ago

To add some detail: a common minimum quality criterion for optical surfaces is λ/4 (divergence from an ideal surface), where λ is the wavelength used. For visible light this gives about 100 nm (though large professional telescopes are "figured" to more like λ/20). Any alleged glass creep visible in window panes over a few hundred years would quickly ruin any astronomical mirror (which definitely does not happen).

Traster 6y ago

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...