4 ms·

I think while the smaller mirror here is Zerodur, the larger one is just borosilicate, https://astro.arizona.edu/news/rubin-observatory-achieves-another-major-

• IAmBroom 1y ago
  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.