[ATM] Does glass retain moisture?

["john sherman" <atm@mail.johnspics.com>]

Hi List, 

I have a question for the physics folks out there. First, the background. I have read, here and elsewhere, that a Newt's primary mirror is always a parabola irregardless of the current temperature(assuming it was figured perfectly). At 10 degrees F the mirror is physically smaller than it is when the temperature is 60F. Yet the figure is supposedly still a parabola, just the focal length changes a tiny bit after the mirror as a whole equilibrates to a different temperature. Given that, why do some people with mirrors of very high quality claim that the star test is different at warm temperatures than at cold temperatures?? (Of course, I'm talking about AFTER the mirror has equilibrated to the ambient temp. We are fully aware that the star test is horrible AS the mirror equilibrates.) 

So, I note that warm temperatures are usually encountered in summer, when there is more moisture in the air (higher dew point). Normally, the air is relatively very dry when cold temperatures are encountered (most of us only bring our scopes out when it's clear). So the question: Does glass retain moisture from the atmosphere into the gaps in its molecular structure? 


For example, I recall a thread where it was pointed out that "waterproof" dental stone does not "absorb" water, but water molecules do get into the material. The water molecules do not bind to the material, but they can get into the gaps between the molecular structure of a material. So, if moisture can get into a piece of glass, does it push apart the molecules of the material, thereby causing the piece of glass as a whole to expand slightly? 

If a piece of glass changes size slightly depending on different amounts of moisture in the air, shouldn't that affect the figure enough so that someone with a very finely figured mirror can detect the difference in a star test? The edge of the mirror is thicker than the center, so it seems reasonable (to me) to assume that the edge would contract more than the center in the low humidity of winter, giving the impression of a tiny bit of over-correction. The tiny bit of overcorrection is what is observed in the cold, if perfect correction is observed in the warmth. 

If perfect correction is observed in the cold (winter, low humidity), then undercorrection is observed in the summer (warmth, more humidity). As the mirror retains moisture it expands slightly, and the edges expand most - deepening the curve. Of course, someone with a quarter-wave mirror is not likely to see any of this, it requires a very well-made mirror. 

Does any of this make sense to the 
physicists out there? 

Thank you, 

John Sherman 

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