3 ms·

Because glass does not sagg. It is a myth. https://en.wikipedia.org/wiki/Glass#Behavior_of_antique_glass https://en.wikipedia.org/wiki/Glass#Behavior_of_antiq

• gattr 7y ago
  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-mirror-cell-question/ https://www.cloudynights.com/topic/562906-18-point-flotation...