Re: [ATM] Coolling fans for mirror

["rugo@iinet.net.au" <rugo@iinet.net.au>]

Thanks Dale for your advice.  It makes good sense.
Rugo

On Sat Jul 28 23:31 , Dale Eason  sent:

>Usually we want to cool the mirror to ambient as fast
>as possible so that we can get the best view soon.
>There are other temperature effects to worry about
>besides just cooling the mirror, but if the mirror is
>at ambient then those will be controled as well.
>  
>
>Cooling is proportional to air flow over the surface
>of the mirror.  The purpose of the back fans is to
>cool the whole mirror as fast as possible.  I have
>seen reports on the ATM lists based on fluid dynamics
>that blowing directly on the back of the mirror is the
>most efficient method.  It does not matter if it
>creates cooler spot because they will even out once
>the mirror as been cooled to ambient as best as
>possible.  The more air you can move over the back the
>better so in general the bigger the fan the bigger the
>volume of air.
>
>Sucking air out the back is not the most efficient
>because the air stream tends to form at the edge of
>the mirror and so does not form across the surface. 
>Thus there is less surface area involved in heat
>transfer.
>
>Sometimes there are other things to consider like
>sucking up dust an blowing it into the tube.  There
>are always trade offs and one must balance the design
>to meet their own conditions.  Overall a single small
>fan supported on rubber bands aimed at the back of the
>mirror is a very good way to cool the mirror quickly
>and has been used successfully by many.
>
>
>Dale Eason
>
>--- "rugo@iinet.net.au" rugo@iinet.net.au> wrote:
>
>> Hello all
>> A part of refurbishing a 20 inch scope I need to fit
>> some fans at the back of the 
>> mirror cell to help air circulation of the mirror. 
>> I have two questions.  Is there 
>> a danger of an air stream from fans blowing direct
>> on the back of the mirror 
>> causing localised stressing.  The stressing would be
>> the result of the fan's air 
>> stream cooling small areas of the mirror.  (The end
>> result would be image 
>> distortion.) If so, would it be better to have the
>> mirror surrounded by a large 
>> generalised  volume of circulating air?  The fans
>> would need to be  baffled in some 
>> way to prevent the air stream blowing directly on
>> the mirror.  I would like to hear 
>> your views.   
>> Clear skies.
>> Rugo
>> 
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>
>
>
>       
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