From: Mark Holm 
To: "Jeremiah J. Burton" 
CC: atm{at}shore.net, 'Gary Seronik' 
Reply-To: Mark Holm 


Jeremiah J. Burton wrote:

> I Appreciate all the help and advice I have received.  It looks like I
> will try and replace the mirror cell on my 8" Meade Dob.  Now, I was
> wondering if it would be better to put a vent with a fan just in front
> of the mirror, or if it would be better to place a fan behind the
> mirror?  If I place it behind the mirror, should it blow in or out?
> Would it be overkill to put in both fans.
>
> Thanks
>
> jeremiah



Alan Alder's work shows that in front is definitely better.  On an 8-inch,
fans in front and in back is probably overkill.

The reason front placement is better is that, while the mirror is still
above ambient temperature, a boundary layer of warm air forms right next to
the mirror
surface.  This layer acts like a lens, bending light that passes on its way
to and from the mirror.  This would be fine if the air lens were correctly
figured and uniform, but it is most definitely not.  The boundary layer
"lens" accounts for the greatest part of what have long been
referred to as tube currents. Since the boundary layer is thin and quite
close to the mirror, it is difficult to sweep away.  (The viscosity of air,
though low, is enough to stabilize thin layers close to an object.)  Slow
air movement doesn't do the job.  You need the
rapid movement from a fan blowing directly across the mirror surface to break it up.

With the boundary layer broken up, the scope will perform much better, even
while the mirror is still significantly above ambient temperature, after
the mirror reaches ambient temperature the fan can be slowed down, or
perhaps turned
off.


The other property of the boundary layer is that it acts as insulation for
the mirror,

slowing heat transfer.  Breaking it up greatly speeds heat transfer from the

mirror to the air, allowing the mirror to come to equilibrium more quickly.


Quite a few ATM's have verified Alder's experiments with their own
practical experience.  I think we can safely say that front blowing fans
are a proven, effective technique and that they are in most cases superior
to rear mounted fans.

One thing that could make the fan not work well is the presence of large thermal
gradients in the air that the fan is blowing into the telescope.  For
instance, a telescope set up on a warm driveway might not benefit as much
from a fan because the air near the driveway will vary in temperature and
the fan will just
blow mixed hot and cool air in a poorly mixed mess.  Even in these
conditions, the fan would help the mirror more quickly come to the average
air temperature.


On a clear night (Most of us don't observe on cloudy nights.), air
temperature typically falls as the night progresses.  With falling air
temperature, the mirror lags behind, staying warmer than the ambient air. 
In this condition, the
  detrimental boundary layer of warm air can persist all night.  Thus it is
often helpful to run the fan, though perhaps at lower speed, all night.

Someone wrote on this list a while back that running a fan also prevented
dewing.  I don't know why this would be so, but I can't argue with a direct
observation.

Mark Holm
mdholm{at}telerama.com

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