Re: [ATM] Does glass retain moisture?

["Lawrence D. Lopez" <lopez@mv.mv.com>]

Wait one cotton picking second:
What would happen if you coated both sides of the mirror with aluminum ?
Would something magical happen ?
Wouldn't the mirror radiate less and thus cool down slower with the
subsquent characteristics of fewer tube currents and less dewing ?
Not that dewing is a problem.

Just a really weird idea

    Larry

Mark Holm wrote:

> 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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