Re: [ATM] Fan options

[Mike Lockwood <melockwo@uiuc.edu>]

Rod,

Rod Brackenridge wrote:
> Thanks for that.  I did look closely at your cooling system because
> it's the same size mirror. It would be awkward for me to place that
> many fans together at the front without relocating the truss
> clamps.  Is there any problem with placing fans in the corners
> blowing diagonally across the mirror surface?

Yes, I overdid it with the fans on this scope!

My point (for all others reading this, too) is that every mirror box 
is unique, and it is difficult to model air flow.  I use fans blowing 
diagonally across the front of my 16" F/4 mirror, and they do help 
cool the mirror.  (There are 2 more blowing on the back.)  Is there a 
better arrangement that might cool faster?  Quite possibly.

My advice is to experiment a bit.  Do some star testing with a warm 
mirror.  (Well, this may or may not be easy this time of year for 
you.)  Observe the air currents from the mirror.  Try different fan 
positions.  See which configuration seems to most uniformly "scrub" 
the thermal layer off the surface of the primary.  Then duplicate that 
configuration on the back of the mirror, if possible.

Again, once you are moving a certain volume of air uniformly around 
the mirror (including the edges, if possible), you can't do any better 
due to the thermal conductivity of glass.

	Mike Lockwood

> ----- Original Message ----- From: "Mike Lockwood"
>> I'd encourage you to cool both the front and back of the mirror.
>> I have a heck of a time cooling off the full-thickness primary of
>> my 12.5" F/12.5 Cassegrain.  I am using six 75mm
>> double-ball-bearing fans, two on the back, four across the front.


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