300mm dob cooling fan

[Blackheart]

Hi guys

Been looking through the site but I can't find a thread with any advice on what kind of cooling fan might be good for my SW 300 flextube. Since I bought it two years ago I have never had a night of 'good seeing':( Then I was reading about the thermal boundary layer at:

Beat the Heat: Conquering Newtonian Reflector Thermals ? Part 1 | Gary Seronik.com

I was really interested in this information because I had always expected sharper views from my scope. I suspect that my collimation isn't perfect but I am now convinced that thermal disruption might be a large part of the problem.

Could anyone point me in the right direction for getting a suitable cooling fan...

many thanks

BH

[Blackheart]

It's ok guys...

Found a 120mm fan on ebay for £2 and also found this website...

Telescope Optics Topics

Now I just have to think about the best power source!

Cheers

BH

[spaceboy]

Do you need a cooling fan on a flex tube ???? Give the front of the mirror a quick blast of with a rocket blower is my thinking ???

[Blackheart]

I'll let you know when I fit it Spaceboy. All I know is there must be some explanation for consistently bad seeing. For tiny fleeting moments the view occasionally sharpens so it can't just be collimation although I know this is not perfect. Here is the main link that convinced that a fan might improve the view...

Beat the Heat: Conquering Newtonian Reflector Thermals ? Part 1 | Gary Seronik.com

Cheers

BH

[spaceboy]

Do you use a dob cloak and what EP's do you have ??

[umadog]

Do you need a cooling fan on a flex tube ???? Give the front of the mirror a quick blast of with a rocket blower is my thinking ???

You certainly do need a fan. A big chunk of glass stores a lot of heat. A rocket blower would be as much use as a peashooter on an elephant hunt.

What you're describing as fleeting moments of clarity sounds like bad seeing. Two ways of tackling it are to wait until later in the night to view, try to observe when the object is near the meridian, and try to avoid looking over nearby rooftops.

With a fan, a 12" mirror should get down to ambient within an hour or two, but obviously it depends on the temperature differential. You can tell if the mirror is down to ambient by defocusing on a bright star until you see a large disk. A warm boundary layer looks like a bunch of slowly moving wave lines. Those are the convection cells. When the mirror is cool the defocused image should look totally steady. Put your hand over the front of the tube and you'll see a heat-plume come off it. That plume is a lot less obvious when the mirror is warm.

You should also be certain of your collimation.

[umadog]

One thing you can try is to e-mail a few of the premium telescope makers, such as Starmaster, Starstructure, or Webster, and ask them what fans they're using. I know Webster have gone to significant lengths to find fans that don't vibrate too much.

[spaceboy]

For what it's worth http://stargazerslounge.com/beginners-help-advice/158906-can-i-have-some-members-cooling-fan-solutions-please-3.html