Re: [ATM] Coolling fans for mirror

[Dale Eason <atmpob@yahoo.com>]

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
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



       
____________________________________________________________________________________
Sick sense of humor? Visit Yahoo! TV's 
Comedy with an Edge to see what's on, when. 
http://tv.yahoo.com/collections/222
_______________________________________________
ATM mailing list http://www.atmlist.net/