Re: [ATM] Fan options

[Mike Lockwood <melockwo@uiuc.edu>]

G'day Rod,

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.

Just keep in mind that you can only do so much good - once you get a 
good stiff breeze going and fairly well distributed over all surfaces 
of the mirror, then the thermal conductivity of the glass will limit 
how fast it can cool off.

I just finished a 13.1" x 7/8" Pyrex mirror - should be lots of fun to 
take come cooling data this winter (take mirror from 68F house to 20F 
outside air, see how fast I can cool it!)

	Mike Lockwood


Rod Brackenridge wrote:
> I want to add fans to my 12 inch dob.  I have a full thickness (2
> inches) pyrex mirror.  I have read Brian Greer’s A&T article and he
> recommends a fan/s with 20 to 30 cfm for my size mirror and
> suggests both front and rear cooling in about this size and larger.
> Mounting several fans together in a bank as I have seen some do is
> difficult with my scope as the truss clamps would be in the way.
> I was wondering which strategy would be better:
> 
> Option 1: Placing a larger (100mm) fan mounted on the rear and the
> following fan blowing up across the front:
> http://www.pccasegear.com/prod249.htm
> 
> Option 2: Mounting 2 small cpu fans in the mirror box’s bottom
> corners blowing diagonally across the mirror front.  Again a 100mm
> fan would be mounted to the back.
> 
> Any comments or suggestions appreciated.
> Rod Brackenridge Mt Martha AUSTRALIA



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