ATM Adding a fan to an SCT - some lessons learned

["Tom & Lou Krajci" <krajcit@3lefties.com>]

I have added a small fan to the back of my C-11 OTA.  My main goal was to
improve cool down time and reduce tube currents/temp differences in the OTA
while observing.  Yes, the fan is intended to run at a lower speed while
observing.


It's a fairly small fan, a bit over two inches square, runs on 12V DC.  I
had to cut a small hole in the rear casting, and four mounting holes.  I
also cut four holes (two inch diameter) in the part of the tube about an
inch back from the corrector plate.  These are covered with filter material
from a HEPA vacuum bag.  (NOTE:  When you cut up the vacuum bag, discard the
outer/stiff/paper portion.  You don't need to withstand such a strong
pressure from a small fan, the paper restricts air flow, and I don't thing
it helps filter any better.)

My biggest fear was that the fan would vibrate the scope enough to spoil the
image while observing.  So far, in moderate seeing, at about 350-400x there
may be the slightest bit of vibration visible at the eyepiece when the fan
is running at 12 volts.  When I slow the fan down I can't detect any signs
of image vibration.

I've made plastic covers to protect the inlet vent holes, and another cover
to place over the fan when it's not in use.

Even though I made four inlet holes, the HEPA filter material still reduces
air flow considerably.  If I were to do it again, I'd make the inlet vent
area greater to allow easier air flow.

OK, so how does it improve optical performance?  I can't say yet, more
testing needs to be done in better seeing, and comparisons of fan on/fan off
optical performance.  I'll post more when I learn it.

Tom Krajci