So it sound like plate is a good substrate, and FWIW I'm more referring to more common sizes like 6" to 16" just to throw out a ballpark. I wonder too whether such mirrors are more sensitive to temperature effects when placed in a closed tube with poor mirror cell ventilation. It may be that smaller plate glass mirrors are more susceptible to temperature effects during figuring due to heat transferred from the optical worker. In usage the thin mirror stabilizes quickly and does not appear to suffer unduly from temperature changes-although I have not used it yet under rapidly falling temperature conditions. Under these conditions we never noticed any transient effects on the mirror during testing that would have suggested a need for a multi-hour long delay. It was not rinsed in cold (or warm) water. The mirror was wiped with a sponge and immediately dried with a paper towel after figuring and before testing. There was a waiting period of about 20-30 minutes between the end of each figuring session and the testing session. Or said another way, the heat that was transferred was spread over a large area. The mirror was large enough that my hand contact did not transfer appreciable heat to the substrate. My figuring sessions were generally short-5 minutes or so. Having recently finished a 14" x 1" plate glass mirror I'll chime in on this question. The main reason low and zero-expansion materials are used for mirror making is that they are much easier to generate a good surface on them on the part of the optician. Until the mirror stops heating or cooling the air around it, it cannot perform at it's best. But once a mirror is made with a good surface on it, there is no reason to expect a difference in the views between a plate glass and a low-expansion glass mirror once both are stablizied at ambient temperature. Even low expansion glass is tough to make into a truly good mirror once you get above a certain aperture for the same reason. Moreover, in an mirror cell that is open to the air, the mirror will change it's shape enough to be noticeable in the eyepiece while cooling down. Plate glass might also have internal stresses locked in it too, which like any other glass requires it to be annealed before grinding. That means you'll have to wait several hours between polishing sessions with a larger plate glass mirror to be able to tell what progress you've made. It is subject to problems that low-expansion glass doesn't nearly as much suffer from due to expansion and contraction during polishing and figuring. Plate glass will work well for mirrors, at least the smaller ones.
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