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Re: [ATM] Does glass retain moisture?
On Fri, 2 Apr 2004, Mark Holm wrote:
>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.
Vapor deposited aluminum has an emissivity of about 0.02, while the
various glasses commonly used for mirrors have an emissivity of about
0.95. The aluminized surface of a mirror has substantial impact on it's
cooling properties.
>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.
Perhaps. I suspect it depends on the telescope. In cassegrains, the front
of the mirror can't radiate because it is aluminized. The back of the
mirror radiates to the housing. Newtonians with open mirror cells can
radiate much more efficiently.
I doubt that conduction is significant to mirror cool-down, since the
contact surface area is relatively small and glass in an insulator.
Convection is probably more efficient than conduction.
>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.
Yes, but these layers will be really, really thin. I doubt it has a
noticable effect.
--
David Whysong dwhysong@physics.ucsb.edu
Astrophysics graduate student University of California, Santa Barbara
My public PGP keys are on my web page - http://www.physics.ucsb.edu/~dwhysong
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