Cooling Fan -Suck or Blow ?

[gfletcher]

Hi Guys

Having recently acquired a secondhand 250PX i am a little unsure as to whether to mount the fan on the bottom of the OTA to "suck air out of the tube or blow air into the tube" !!

There seem to be two train of thoughts on this.

Can anyone enlighten me as to which method would be the best to improve the viewed image.

thanks

Graham

[Bizibilder]

I would go for "suck air out" - this should give an even flow over the mirror and cell, I would assume that "suck in" could create a "cool spot" on the mirror. Secondly a fan "sucking in" from low down ie near the floor could well pick up dust and grit etc - not the best stuff to get in a 'scope tube.

Just my tuppence worth!!

[tooth_dr]

Blowing up the tube is how I fitted the fan on my 250px. It seems to be the general conscensus that this forces air up the tube, cooling the mirror and removing the layer that forms directly above the mirror. The bottom of my OTA, being on a GEM, is never near the ground, this might be different for a Dob.

[Bizibilder]

This is going to be one of those 50:50 answers!!!

[tooth_dr]

To quote an article by Bryan Greer:

"When using a rear-mounted fan, you have a choice of airflow direction; blowing onto the back of the mirror, or sucking air out the bottom of the tube. Unless the bottom of your tube is closed and carefully ducted, blowing the air stream at the mirror will be more efficient (i.e., a smaller fan can be used to get an equivalent cooling effect). In practice, this slight advantage is almost meaningless, and other more pragmatic considerations take precedent. If you regularly observe from a dry, dusty, location, consider pulling air out the bottom of the tube to reduce the accumulation of dirt on your optics."

[ollypenrice]

Don't mess about here, go the whole hog! One of my guests made a watercooling jacket for the bottom of his mirror with a spiral canal inside to pump water evenly around the lower surface of the glass. Three temperature sensors on the mirror back and several more in the tube keep him up to date with what's happening via a set of dials on the outside. Get busy, shouldn't take you long!!

Olly

[gfletcher]

"Priceless Olly"

[davew]

There is another one that was proved to work very well. In fact better than some rear of cell blowers.

Blow across the face of the mirror from the side of the tube. If well filtered it kills the tube currents dead in seconds. I can't remember where I saw it now but it looked very powsh. I think it also had a vent directly across from the fan.

Dave

[skye at night]

For a newt or other 'open-ender' a suck out fan would be best I feel..

For closed tube.. ie, corrector plate at front like Schmidt Newt, Mak or SCT a pair working together with one extracting and one intaking would work well.

Steve

[brianb]

Blow air in ... through a filter.

[Astro_Baby]

Rather pointless trying to 'protect' the mirror in my view. The whole thing is open ended anyway with dew, bugs and general grot blowing in with the wind.

I have my fan blowing air in from the back to provide a coolant for the rear of the mirror. After all its less likley that any grot will make its way round the system and onto the mirror or at least no worse than sucking air in from the front of the tube where air will be neatly inhaled straight onto the face of the mirror.

Put a big enough fan in thats vibration free enough and just blowing air in from the back will disrupt any warm air on the mirror face.

[davew]

Found one example:-

http://www.pha.jhu.edu/~atolea/WAS/thermal_management_newtonians.pdf

It gives some idea of what goes on even if you disregard the side fan idea.

Dave