Need advice for installing fan on a dob

[zoneca]

I'm confuse in installing a fan for my 12" dob, whether facing the blowing in or out? If blowing in the mirror will be colling faster but I'm afraid the dust will easily stay inside the primary mirror. If blowing out, it would take longer time in colling the mirror & could be just a waste. So what should I do?

[Moonshane]

I have mine blowing on the back of the mirror. to reduce dust, put a filter on the fan but I don't bother.

[zoneca]

Thanks Moonshane, I guessed not many people attached a fan on dob, that's why only you alone replied my question. It means only both of us having dob with additional fan... lol.... unbelievable. ..

[jimmyjamjoejoe]

I have mine the other way, blowing air out the bottom. There really isnt much difference, but having it baffled makes a big difference.

[sparkyjon]

Im planning on it. I have 40mm 5v for the secondary mirror with a battery pack switched and Im planning on getting a 200mm for the rear of the primary 12v and another battery pack..just need a big dob now lol.

Its probably not needed but i think the cooler the better.

[Moonshane]

I'd not bother with a fan on the secondary as this is very likely to introduce vibrations. some people heat the secondary but I have never bothered. you are right with the cooler the better to a point but really it's more accurate to say, the more equal the better. the idea being to get your scope to ambient temperature and to stay with any changes that happen during the night. e.g. if you take a scope from a cool cellar out on a warm summer evening, there's a chance the scope will initially have to warm up not cool down and then over the night cool down as the night gets colder.

[jimmyjamjoejoe]

There is also a limit as to how big is useful. The cfm needed to shift the maximum amount of heat from the mirror is surprisingly low. Unless your mirror is over 20", a 10-12cm fan running on low voltage is more than enough, strapping a 25cm fan on full power wouldnt cool it any quicker.