Re: ATM Baffeling ventilation

[Mark Holm <mdholm@telerama.com>]

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@telescope