Re: [ATM] fans

[Mark Holm <mdholm@telerama.com>]

John Sherman wrote:

>I had very little trouble keeping the 2.2" Pyrex mirror within one degree of ambient. Here's a secret: Close up your tube with a shroud and so forth. Do not expose your mirror directly to the ground or the sky. And also protect your ambient-air thermometer from the ground and the sky. 
>
>For example, if the thermometer is exposed to the sky and the mirror is not, the thermometer will generally read a few degrees colder than the mirror. You can make the mirror warmer than ambient by exposing it to the ground. Experiment, and you will see what I mean. Exposing a mirror to both the ground and the sky sets up a continuous thermal gradient in the glass and makes controlling the temp very tricky. It might even give astigmatism to your image. Try to measure the temp at the bottom and at the top of the glass.
>

These seem like very important and useful observations.  Good job, John!

Your observations about ground exposure vs. sky exposure confirm 
predictions based on radiative heat transfer.  Shielding from the ground 
is a new, and possibly quite important suggestion.  It could also keep 
out dirt, etc.  I have seen pictures of scopes with cloth covers over 
the back of the mirror box.  These were intended to keep out dust, 
spiders, etc. but would also shield from radiative heat from the ground.

>Since the front of the glass is insulated with aluminum the fan takes forever to cool the mirror.
>
I have to quibble a bit here.  It is hard to argue with first hand 
experience, but I can't see a reason that aluminum would insulate 
glass.  Aluminum has approximate 200 times better thermal conductivity 
than glass.  The vacuum evaporated layer of aluminum has to be in very 
intimate contact with the glass.  There shouldn't be an appreciable 
barrier between them.  Now the surface of the aluminum is oxidized.  The 
listed thermal conductivities for aluminum oxide vary by a factor of 10 
or so depending on physical form (crystalline, amorphous, etc.), but all 
of them are higher than the value for borosilicate glass, though lower 
than metallic aluminum.  Is there another way to make sense of this 
observation?  Maybe, because of the thinness of the layers, there is a 
weird quantum mechanical effect???  Maybe it just has to do with how the 
air actually flows over the surface in your particular setup.  Perhaps 
your front fan isn't actually strong enough to blow all the way through 
the boundary layer?  Air gets pretty viscous up close to a surface.

-- 
Mark Holm
mdholm@telerama.com


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