Re: ATM Adding a fan to an SCT - some lessons learned

[astrojoe@erols.com]

Hi Tom,

I did this with my C 9.25" a few years ago. I only drilled out the rear
casting to have a filtered intake. I mounted a small fan to a piece of
delrin I machined that slipped into the 2" rear back. Turned on the fan
to cool down the ota in 20 minutes. Without it, cooldown time was 2.5
hours and counting. I am sure you have experienced this. The temperature
delta was large of course in the winter time and in the summer it was
still at least 1.5 hours of cooldown time. The fan helped the optics
reach their optimum performance ( nearly perfect inside and outside
images ) level quickly. Celestron should make it standard on all their
closed tube ota's.

As far as the optical performance getting better ? Yes, it went from
unusable to working just fine. Having the fan running during viewing was
impossible in my setup. Lets see what you find out on this. I doubt if
it will make any signifacant difference visually.

Click on this PIC link to see the fan setup. You will have to page down
past the other pics of the brass telescope, 4" Florite Myiauchi Binos,
etc. 

http://www.myhomeis.com/jcscope3/


Joe Castoro








Tom & Lou Krajci wrote:
> 
> 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