Cooling Fans

There was a thread a while back regarding (in part), cooling fans. There were various suggestions regarding placement and flow direction etc. As well as observations that noted adverse currents affecting the image almost as soon as the fans are switched off. So my question is, does this mean all of you keep the fans running even when you're actually observing? Or do you switch them on and off between view(s)? I've got a solid tube dob (10" f-7) that could really use the help of some fans, but I'm concerned about visually noticeable vibrations from the fans, especially at higher magnification(s). Any thoughts on this?...Clear Skies!
Rich

Hi Rich,
I make fan systems and other accessories, Blue Sky Accessories.
I have made many fan systems for scopes from 8" to 40", with up to 6 fans, and they all use them while viewing. If the fans are mounted properly, then vibration is not noticeable or non existent.

I invariably find better optical performance by
leaving the fans running while oberving, though
often best performance is at lower fan speeds.
For example, using high speeds to quickly cool
the socpe before observing, and then low speeds
while observing.

As previous posts stated, vibration will not be
a problem if you use good fans and mount them
properly. There seem to be a variety of successful
mounting strategies. I find that suspending the
fan somewhat loosely on rubber bands has always
given good results -- for example locating four
studs about 2 inches from the fan, and then lacing
rubber bands between the holes in the fan corners and
studs, so that the fan does not touch anything except
the rubber bands. Mounting it on a flexible foam
sheet, like the stuff used for camping sleep
pads, also seems to work well -- i.e. using a sheet
of this stuff and then mounting the fan in the middle
of it. Mounting the fan on metal springs also works,
though I have sometimes seen problems if there
was a resonance between a particular set
of springs and the scope. Rubber and foam sheets
have good damping properties, while steel spings do not
provide much damping.

As for the direction of the airflow, I've got an 8"
close tube scope, and I get the best results on that
one by sealing the area around the fan, and then using
the fan to draw air out the back of the scope. This
approach avoids rising tube currents simply by pulling
"clean" air in the front of the scope and sending it
out the back.

For a larger open tube scope (truss Dob, etc.) the airflow
direction answer is probably less obvious, and there
may be some advantage to directing outside (cold)
air directly at the mirror surfaces.

As for the type of fan to use, you certainly want
a good-quality low-noise (low vibration) fan. I've
had good luck with the Sunon Magnetic Levitation
fans,
http://www.alliedelec.com/catalog/pf.aspx?FN=790.pdf
but I've not done any extensive comparisons to
other brands. High-quality models claiming to be
low-noise or ultra-low-noise might be a good place
to start. They've probably paid some attention to
smooth bearings and well-balanced blade assemblies.