[ATM] Forced Ventilation

["Doug Williams" <salguod@minbaritm.com>]

Hi all,

 

I recently closed the back of my solid tube Dobsonian and installed a fan at
the bottom of the tube that sucks air from the tube and out behind the
mirror.  Because the back of the tube is closed, this results in forced
ventilation with air moving down the tube, past the mirror, and out.  I know
that this is nothing new or revolutionary, but I wondered how many other
people are using this kind of system and how satisfied they are with the
results.  Here is my view on this, please chime in if you agree or disagree.
My experience is with a tube, but we are building a truss now, and I wonder
if we can get the same results by closing the bottom of the mirror box and
using a shroud to act as a "tube".

 

Cons:

1.  The one obvious disadvantage is that now that your tube is closed, you
must operate the fan continuously.  If you turn the fan off, the mirror will
not cool well.  Not that big a deal, since many scopes require constant
power anyway, and the fan does not draw much.

 

2.  The other possible disadvantage is possible vibration from the van
during observing.  But I am convinced that my vibration isolation system is
working perfectly, and I can see no vibration at the eyepiece even at
extreme high power.

 

Pros:  There are several.

1.  Since air is moving past the front AND the back of the mirror, the
mirror cools quickly from BOTH sides using only one fan.  This greater
efficiency allows for a smaller fan with less vibration and current draw.

 

2.  Air blowing down the tube breaks up the thermal boundary layer on the
surface of the primary mirror.  In systems with an open back and the fan
blowing on the back of the mirror, most of the air just blows back around
the fan, instead of going PAST the mirror.  There is also some
recirculation.  In a closed tube system such as mine, new air from the top
of the tube is continuously being brought in without recirculation.

 

3.  The air is coming from the TOP of the tube.  It is usually cleaner and
cooler than air near the ground, and in any event, the temperature more
accurately reflects the temperature we are trying to observe in.

 

4.  Air is drawn past the secondary mirror, as well as the primary.  This
cools the secondary mirror to ambient temperature, and (in my experience)
also vastly reduces or eliminates dewing without having to heat the
secondary.

 

5.  With a closed back tube, you don't have to worry about stray light
getting to your mirror from the bottom end of the tube.

 

Thoughts?

 

Doug

 

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