Re: [ATM] basic questions about cooling fans

[Mark Holm <mdholm@telerama.com>]

Hello Marro,

First, there are two excellent articles in the most recent issues of Sky 
& Telescope on the use of fans for cooling telescope mirrors.  Look in 
the May and June 2004 issues.

>Ten years ago I built a 12,5". So far I never used a cooling fan.
>I understand those are necessary to cool down the primary. 
>1 - does the fan has to operate continuously or only in temporarily (and
>swithed off after a while)? 
>  
>
This will depend on particular circumstances.  With a large thick 
mirror, the fans may need to run continuously.  It also depends on the 
air temperature during the night.  If the air temperature continues to 
fall during the night (as it often does), then the fan may be needed to 
keep the mirror temperature close to the falling air temperature.  If 
the air temperature stabilizes, then the fan may not be needed after a time.

>2 - I would expect that the air stream causes bad seeing. is that so?
>  
>
Moving air causes very little image distortion.  In fact, movement by 
itself, causes no image distortion.  Pressure variations that usually 
accompany movement cause a little image distortion, but this is a small 
effect.  What does cause a lot of distortion is air with different 
temperatures.  It is almost always better to move air in order to get 
more uniform temperatures.

Very often, a layer of air near the mirror becomes warmed when the 
mirror is warmer than the outside air.  This layer is called a boundary 
layer.  If the boundary layer had a nice uniform and optically correct 
form, it would not harm the image, but that is never true.  In fact, the 
boundary layer often causes the most damaging image degradation of 
anything in the light path.  Mixing up this boundary layer by blowing a 
fan on it can improve the image very rapidly, even when temperature 
differences are still present.

>3 - what king of cooling fan works best? internal PC-fans? I prefer to use a
>batterypack of only 3 or 5 Volts - is that possible? 
>  
>
Electronic supply company catalogs list a lot of 12 volt fans, but there 
are 5 volt fans also.  I don't know if there are any 3 volt fans.

>4 -and about the upcoming Venus transit: is a cooling fan especially a must
>have???
>

I don't have enough experience with solar telescopes to know.  I do know 
that a white painted telescope is best, and it will usually be best to 
observe over a grass field, or perhaps a lake, but certainly not over 
pavement or buildings or dark plowed fields.

Mark Holm
mdholm@telerama.com

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