Re: [ATM] fans

["john sherman" <atm@mail.johnspics.com>]

Hi Mark,

>>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.  

If all you're gonna do is quibble, then I did better than I expected!

I was thinking of how a Mylar emergency blanket works. The Mylar does prevent airflow, trapping air. But I understand that the aluminum coating is most of what keeps you warm. But your point is right, the aluminum is in direct contact with the glass in our case, so more conduction occurs, the heat then radiating from the aluminum surface. 

I suddenly wish for thermal photography capability. Would someone put a warm mirror on its edge in a cold room and see if more heat leaves the front or the back?

>Maybe it just has to do with how the air actually 
>flows over the surface in your particular setup.

A definite possibility. The fan on the front has to blow from outside of the light path, so it's hitting the surface of the mirror at an angle. I'm guessing that lowers its efficiency. To increase efficiency I open the shroud opposite the fan to let out warm air, and usually used the exhaust fan at the same time. So very little of the wind from the front fan hits the back of the mirror. Whereas, the rear fan hits squarely on the glass, and then blows around to the front so it cools both sides of the mirror at the same time. And of course, the front of the mirror, being smooth, has less surface area to radiate from.

>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.

This could be the case here. It might escape detection when looking at the seeing with an out-of-focus star. 

Thanks for writing,

John




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