Re: [ATM] Does glass retain moisture?

[Mark Holm <mdholm@telerama.com>]

John Sherman wrote:

>Perhaps the aluminized side of our Pyrex disks perform the same function? The back side and the edge of the glass could absorb more surface moisture than the front side. If it works the same as my flimsy example, in the summer when there is more moisture the figure on the mirror would be less corrected, more concave.
>
>Does that sound feasible?
>
>  
>

I have never heard or read of this sort of behavior from glass.  The 
surface moisture on glass is  really only at the very outside, and 
doesn't penetrate enough to have a mechanical effect, or at least the 
effect is pretty small.

Aluminizing one side should change infrared emissivity some, so if you 
had an environment where radiative heat transfer was the dominant mode 
effecting the mirror, the front and back could get to different 
temperatures.  Radiative heat transfer surely plays a role in telescope 
cooling, but I think there are usually stronger sources of heat transfer 
that tend to mask it.  Also, a telescope out on a clear night does not 
have a uniform radiative environment.  The sky is much colder than the 
ground.  That asymmetry is at least as important, and perhaps more so 
than the asymmetric emissivity/reflectivity of a one side coated mirror.

BTW, the aluminum layer gets a coating of aluminum oxide naturally on 
the surface from exposure to oxygen.  (This is a good thing.)  Aluminum 
oxide will pick up a surface adsorbed water layer similar to, though 
perhaps not equal to, the layer on glass.

Mark Holm

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