lew@ihlpa.UUCP (Lew Mammel, Jr.) (11/05/86)
There have been a number of anecdotal reports of sagging glass windows. It occurred to me that large telescopes would make a good test case for this purported phenomenon. The Mount Palomar mirror is old, massive and its shape is subject to exacting scrutiny. If a window can develop a "noticable" shape change in a few decades, I should think the same phenomenon would be well observed at Mount Palomar. If noticable means 1 mm, then the Hale telescope (It's at Mount Palomar, right ?) is sensitive to changes one ten thousandth as great. So if it sagged 1 mm in 50 years, the optical quality would be affected in a few days. I know that its elastic response to gravity induced stress has to be dealt with, so what about its inelastic response? Lew Mammel, Jr.
morse@leadsv.UUCP (Terry Morse) (11/07/86)
If you have any doubts that glass flows at room temperature, look at its
viscosity in the CRC Handbook. The viscosity is very high at room
temperature, but it is *finite*. Thus glass can flow a very slight amount
over a long time at room temperature.
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Terry Morse (408)743-1487
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