From: Mark Holm 
To: atm{at}shore.net, ea8yu{at}arrl.net
Reply-To: Mark Holm 


Goran Hosinsky wrote:

> But if you put a ventilator or two in the bottom of a baffled tube, will
> you not get turbulence around the baffles?
>

Hi Goran,


The thing you need to keep in mind is that the big enemy causing poor
"tube seeing" is temperature variation not turbulence.  Amateurs
have not always been clear on this point, and there was a time when even
the pros were not clear on it.


Since a telescope is filled with air, the only thing that can distort the
path of light through it is refractive index variations in the air. 
Neglecting humidity variations, the only thing that can produce RI
variations is density variation.  And, only two things can produce density
variation: Pressure variation and temperature variation.  Of the two,
temperature variation is the more powerful effect.

Now, in principle, turbulence in thermally homogeneous air produces
pressure variation and thus RI variation.  But, it takes pretty powerful
turbulence to produce enough RI variation to have an observable effect. 
You would need to get
a pretty stiff breeze flowing through your scope.

Temperature variation is much more insidious.  It only takes a 1 or 2 C
difference in temperature to make a very noticeable effect on refractive
index.

Thermal gradients in air dissipate slowly, while pressure gradients
dissipate quite rapidly (unless continuously produced by a fan).  Even in
the flow downstream of a fan, pressure gradients damp out pretty quickly.

To be technical about it, it is unlikely that most telescope tube
ventilators will produce enough air flow in the main portion of the tube to
cause turbulent flow, even over as unstreamlined an object as a baffle. 
The flow will probably be laminar.  Now laminar isn't necessarily better,
you get pressure gradients in
laminar flow too, and they are more organized in both a spatial and
temporal sense than the pressure gradients in turbulent flow.  Still, the
flow speed in a
telescope tube is unlikely to be great enough to produce pressure gradients
large enough to result in objectionable refractive index gradients.

Combining these facts with the observed fact that telescopes and their
optical elements are often several degrees away from air temperature
(especially early in a typical evening observing session), temperature
variation is usually far the greater of the two evils.

The following two Sky & Telescope articles deal very clearly with the effects of
temperature variation in typical amateur Newtonian telescopes.  In
particular, by practical experiment, Alan Adler shows that trading some fan
turbulence for significantly reduced thermal gradients is usually a quite
beneficial choice.

Alder, Alan, Thermal Management in Newtonian Reflectors, Sky &
Telescope, Vol. 103, No. 1, January 2002, pages 132 - 136

Greer, Bryan, Understanding Thermal Behavior in Newtonian Reflectors, Sky
& Telescope, Vol. 100, No. 3, September 2000, Pages 125 - 133


Since the primary mirror is usually the largest thermal mass in a Newtonian
telescope, and since temperature gradients across the front of the mirror
are most damaging, it turns out that air blown across the face of the
mirror is the most effective.  With a solid tube, it may also be useful to
move air through the tube in order to help it come to equilibrium.  As
Alder illustrates, venting
warm air out of the upper surface of the tube can get rid of the greatest source
of trouble without requiring huge volumes of air flow.

A technical note: both Greer's and Alder's articles are illustrated with
color Schlerian photographs of air in front of telescope mirrors. 
Schlerian photos show directly the refractive index gradients in air. 
Alder's photos show distinctly that the thermal gradients are more damaging
than the turbulence from
the fan he employs.  I consider this excellent experimental back up for my
discussion above.

Mark Holm
mdholm{at}telescope

--- BBBS/NT v4.00 MP