Re: [ATM] Fan locations & function for a 32

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Dr. Wm. D. Hanagan, Jr. wrote:
>>1. Should the flow be across the mirror front be from one side to the 
>>other side? Or should it be vertically? (From the lower side of mirror 
>>box, exiting on the top? or vice versa? ) The scope will have a full 
>>shroud as a minimum or possibly a rigid tube later.
>>2. Since its operating in conjunction with the above (and without 
>>starting a debate) should the fans (imbedded in the cell underneath 
>>the mirror) suck or draw the air down and vent out the back of the 
>>box, or blow on the back of the mirror.

To me, this depends on your observing plan.  If you are doing critical 
observing at high magnifications, you may want intervals of heavy 
cooling to remove heat from the glass more quickly, and to drive the 
outer layer of the mirror close to equilibrium.  For this cooling, it 
is difficult to have too many fans, though there is a limit on how 
quickly the glass gives up heat.  Once the mirror is cooled close to 
ambient (at least the outside layer is), then the fans can be turned 
off or their speed reduced greatly for a while until the heat that is 
inside of the mirror migrates out to the outer layer, heats the outer 
part back up and more cooling is necessary.  Of course, the more air 
you blow on the mirror, the more dust goes with it.....

If you are doing more continuous, less critical observing, then a 
method to remove the boundary layer is more appropriate.

> Crossflow Fans
> If you use a rectangular rather than a tube shaped mirror box, or you 
> can otherwise accommodate them, I would strongly recommend using one or 
> more "crossflow" fans. Crossflow fans are designed to produce an airflow 
> pattern that's almost exactly what we need to sweep the thermal boundary 
> layer from in front of a telescope mirror. Here's a link to a ball 
> bearing crossflow fan that is relatively inexpensive when you consider 
> the width of the air stream it produces. The speed of this fan is 
> controlled by the DC voltage applied (between 6V and 12V).
> http://www.coolermaster.com/index.php?LT=english&Language_s=2&url_place=product&p_serial=STF-B01-E1

These are neat.  I've seen one, and held it while it ran.  It seemed 
to vibrate more than a conventional fan, but the air stream it 
produced was HUGE, and nearly ideal for this application.  Just be 
aware that vibration may be an issue, so low speeds might be just the 
ticket for eradicating the boundary layer.  Using two or four in a 
push-pull arrangement would be excellent, I suspect.

	Mike Lockwood

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