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[ATM] fans


Hi Alan,

>I've seen a few sites that say the front fan 
>should blow across the top.

>Where should the mirror be mounted, 

Last night I got another demonstration of the power of a rear fan blowing *up* a solid tube. Racking a star well out of focus showed no boundary layer on either mirror, only the turbulence from the fan itself. For reference, I put my hand in front of the aperture to see the heat waves from it. With the fans OFF, the heat was horribly obvious. With the fans ON, it was a great pleasure to see the heat from my hand blown away. After equalization (an hour, 1.2" Pyrex), with the fan OFF, the only tubulence was from the edge of the field where my body heat got into the light path. 

That is why you should always have a powerful rear fan anytime the scope is pointing near zenith. And part of why I don't care for the front fans (yes, I have one on my scope, I never use it anymore). My advices are for those who have high-resolution scopes, and want high-resolution images from them. Otherwise it doesn't matter much what you do. We do use potentiometers to regulate the fan speed for conditions, as Mark suggested.

YES, if your mirror is not at equilibrium, then a front fan helps clear up the image, as much as it can be with all that heat flying around. But a powerful rear fan also clears up the image, and cools the front surface too. It cools the whole mirror much faster. A cooled mirror does not have a boundary layer, so you can turn the fans OFF and have no turbulence at all (unless pointing near zenith). Isn't THAT the objective?   

This was all done with a 12.5" scope in a solid tube. After some east-coast guy, call him R, ruined the figure on this mirror, it was recently re-figured by our fellow list member Mark Cowan. Star-testing revealed an image very close to perfect - good job, Mark!!

I'm going back tonite for another look!
(how could I resist?)

John






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