silly question

[orion f6.3]

just putting a fan at the front end of the DOB. should the fan draw air out of the tube or blow it in? thanks Orion

[Kaptain Klevtsov]

Silly answer time, it should go on the back, as in lower end.

Heat transfer wise it will be better blowing in, but that will carry the warmer air around in the tube. To draw ambient temperature air into the tube would give better seeing sooner as the convection currents will stop in the tube almost instantly.

Captain Chaos

[Naz]

At the front of the tube, would it not get in the light path?

Also CC, any air currents, even at ambient, would they not still cause problems?

These might sound silly but thats the way I understand it

naz

[orion f6.3]

Ive fitted it to the 11 1/2" dust cover so is full removed when in use. I know the back of the scope is best but orion optics tubes are fully enclosed all bar one 1/2" hole in the center. Ive seen an eyepiece fan on Orion Optics WWW so it look as if they put one at the front of the scope but i was unsure as to blow in to the scope or draw the air out this would suck air in via the 1/2" hole in the mirror base. Orion f6.3 see pics

[Naz]

I think in this case I would get it to suck air out of the tube, blowing in would force dust in and on to your optics. It may be worth while fitting a dust filter over the bottom opening,

naz

[lunator]

Orion

I'd go with drawing air out.

If you blow air in it will compress the air and this may warm the air a bit so slowing the cooling down

Just a thought.

Cheers

Ian

[Kaptain Klevtsov]

Silly question back - how does the light get in the tube?

Captain Chaos

[FLO]

So long as it moves air away from the surface of the mirror - it will work. Generally, a fan works best when placed at the side or rear of the mirror-cell.

[Astroman]

It depends on what you want to do. If the fan is installed temorarily to enhance cooldown, you can put it in either location-top or bottom. It'd probably be best to draw air out of the tube, especially from the bottom. Large, pro scopes are set up for air to flow over the mirror after reaching ambient temp. So, a temporary fan to pull air out during cool down and a fan to blow air over the mirror during viewing is a possible configuration. An amateur here in AZ has such an arrangement on his 24" Truss tube scope. The tubes are covered with a shroud and the mirror box has slits at the basic height of the surface of the mirror. After the scope cools to ambient, he'll turn on the fans that blow across the mirror. Great views through this scope, too! It's a noticable difference, though subtle.

[daz]

I'd draw it out from the top and in from the bottom (my config)

I have toyed with the side fan idea, but at the moment, I'm not brave enough to cut into the tube!!!!!!

[FLO]

After the scope cools to ambient, he'll turn on the fans that blow across the mirror. Great views through this scope, too! It's a noticable difference, though subtle.

This is getting interesting,

How does blowing/air across the mirror improve viewing?

[Astroman]

The theory is, a moving column of air won't allow slow moving temperature differential "cells" to drift across and affect seeing. It's why the seeing is better with a light breeze than very steady, stagnant air. The worst offenders are more or less local, that is low altitude. The lowest layer of air has more effect on seeing than higher up.

You might google "Brian Skiff and seeing" or somesuch. I know Brian's given many talks on this type of thing, and there's sure to be a paper involved, too.

[FLO]

Thanks AM