fans - front or back

I'm sure this has been covered somewhere else but I've not been able to find a good discussion on it.

where is the best place to locate cooling fans for large aperature scopes ... hot air rises but heat radiates ... and it moves in the direction of colder objects.

So is it best to 1) put cooling fans on the back of a mirror blowing toward the mirror cooling the back surface thereby causing the heat on the front of the mirror to move toward the back of the mirror, or

2) place the fans on the back of the mirror blowing away from the back of the mirror thus drawing cool air across the front of the mirror and out the back of the mirror box, or

3) place the fans in front of the mirror blowing tube currents across the front of the mirror and out the side of the mirror box.

I'm at the stage where I can do any of the above ... just don't want to try one and find out one of the other options is definitly better.

Thanks in advance for all your help.

Dan

I'm pretty sure the majority opinion from many here on AM is to have fans blowing across the mirror face and out the opposite side to remove the stagnant air. Then, another fan on the back side of the mirror pulling air out the tube. Some prefer to direct the air against the backside of the mirror for "maybe" quicker results. I suspect either approach is sound, but the first fan across the front seems to be the most important.

Walt

Hi Dan
From the many threads and articals I've read lately about telescope mirror cooling, I have to agree with Walter and Jarad. It seems that the first choice for fan placement would be one or two fans or more (depending on you mirror size) blowing across the face of the mirror to break up the thermal boundry layer and help with overall mirror cooling. The next choice would be to add another fan or two (again depending on mirror size) blowing in on the back of the mirror to aid in overall mirror cooling.
Just my $.02
Buck

Any fan or fans will help. Trying to determine what is optimal with multi fans in place is a bit hit and miss.

I am a proponent of "suck air down" and this has worked well in an 8" tube dob I have. In my 12.5" truss dob, I installed two fans blowing across the primary. I also mounted some fans on thin plywood and secured the wood to the bottom of my mirror box. I tried as best as possible to seal the bottom so the fans get their air feed from above. I also used sorbothane as gasket material for vibration reduction.

Here is a photo of the configuration in the truss dob. This is looking down into the mirror box (sans mirror). Near the top of the opening, you can see the fans mounted to blow across the mirror surface. You can also see the three fans mounted beneath the mirror cell to suck clean air into the mirror box and exhaust out the bottom.

The boundary fans are wired independent of the lower fans. I have a control box with each set of fans on a separate rheostat for speed control. The control box is new and I have not tested it yet. The weather in Nova Scotia recently has been poor and I have not tested this set up since the spring. I have an indoor/outdoor thermometer installed with the probe against the back of the mirror (insulated). Operating the fans at full speed in spring temps required 2-3 hours to get the mirror within 1° C of the ambient air temp. High power views improve tremendously when this occurs.

Gary Weber

Attached Image:

Dan, here’s what I have on my 13" dob. Two 3" Radio Shack fans blowing across (from below), out vents at the top of the mirror box. I control their speed (to tune out vibration) with a 25 ohm rheostat also from Radio shack. I'm planning a third fan on the back, haven't decided yet whether blowing in or out is better. Easy enough experiment, though, just turn the fan around.

The next photo shows the vents from inside and out.

Please excuse the dirty mirror.

Regarding your fan sizes, 4" sounds a little big to me for blowing across, 3" for exhausting out the back sounds a little small. You could probably use 3.5" fans in all three positions to good effect. Probably not critical, tho.

Doug

Attached Image:

I would do both a fan blowing across the mirror surface and one blowing on the back. However you need to make absolutely sure that the fans are isolated from the mirror box. A lot of people use a neophrene mouse pad for isolation but I've seen a lot of cases where that wasn't sufficient.

I ended up mounting a fan on the back and put one in the mirror cover to rapidly cool down the mirror. Once it is near ambient temperature the rear fan seems to do a fine job of keeping it cool.

Ok my $.02 worth.
Suck air out the bottom or blow air in the bottom? Well heat rises right? So blowing in the bottom forces cooler air in the bottom and displaces and forces the warmer air out the top, If you suck air out you are fighting natural convection. Plus blowing air on the bottom of the mirror decreases the cool down time because more air actually hits the surface of the mirror carrying away the heat up the tube.
Blowing across the mirror or not? Blowing across the upper surface of the mirror disrupts the boundary layer which improves the viewing while it carries away the heat and then the blowing fan forces it up the tube and out.
A good filter to stop the dust is also a good idea. You can use electrostatic filter material that does not inhibit air flow or any cloth that lets the air flow easily
Floyd