Re: [ATM] Ball bearing for triangles

["lou krajci" <loukrajci@comcast.net>]

>From: "A. Suijkerbuijk" <a.suykerbuyk@wxs.nl>

>"Classical theory" (books, articles
>etc.) says that the back side cools faster since there is no aluminum on
it.
>Perhaps in the case you mentioned (very thin mirror) things are different.
>Perhaps the mirror will reach the air temperature very fast, after which
the
>aluminum side radiates most. In this case the mirror will cool below the
air
>temperature? Or perhaps the meniscus plays a certain role.

Yes, the aluminized side of the mirror will not radiate as much heat as the
back side of the mirror...but remember that the aluminized side faces the
cold sky, so it has a tendency to cool off...and the back side faces the
earth, which is much warmer than the cold sky...so the back side may stay
warmer because it's facing/exposed to a different thermal environment than
the front side.

This difference in environment is one way to make a cloud detector for your
robotic observatory.  Sandwich a thermocouple between two plates of
aluminum.  One plate faces up to sky, the other faces toward ground.  On a
clear night the upper plate will be colder than lower plate.  When heavy
clouds move in, they reflect IR energy back down and upper plate warms up a
bit.  Measure this temp. difference and you can set an alarm to close the
observatory, or wake the astronomer, because heavy clouds are overhead.

Getting back to the mirror...this is why I advocate multiple fans on both
sides of mirror to help keep thermal equilibrium.

Tom Krajci
Albuquerque, New Mexico
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