Removing boundary layer with fans

[Insomniac]

Hi,
I contacted D. Kriege of Obsession scopes about this, and apart from the issue of not inducing vibration with a fan installation, he said "...laminar flow over the mirror is best but extremely tough to do."

Can anyone tell me why this should be so? After all, he must have seen lots of installations & have a reason for saying this.

Thanks for your thoughts

Chris:question:

[seeker372011]

I don't get it..by definition you will always have a boundary layer in laminar flow-thats why you get a parabolic flow profile? you may get boundary layer mixing in turbulent flow-but you surely you don't want turbulent flow, thats why you want to replace natural convection with laminar flow through some sort of forced convection

if the question is how to create laminar flow in a telescope tube..thats a different question-the gun planetary imagers have some systems in place, I don't really know

but perhaps the best way to reduce natural convection probably is to cool the mirror with temperature sensing and feedback

take all the above with a pinch of salt I have never done any planetary imaging and own a GSO dob with not even a cooling fan so what would I know :)

[bird]

Chris, the better solution is to cool the mirror enough that there is no boundary layer, then switch the fans off while you're observing. I've never had any luck with high resolution observing or imaging with fans running, they always cause too much vibration.

Try installing several high flow rate cooling fans (eg computer fans, 80mm or 120mm or whatever will fit) blowing air onto the back of your mirror and have them run for an hour or so before you observe. Switch them off when you're observing so there is no vibration, and switch them back on whenever you're not observing (this is important, as the air temp will keep falling and you have to keep cooling the mirror).

This will not be perfect, your mirror will still likely be a few degrees C hotter than its environment, but it should be close enough to kill off enough boundary layer effect.

cheers, Bird

[Insomniac]

Great to get your thoughts. I've already got the one fan on the back of the mirror, but am just trying anything that might get that extra bit of steadiness.

I spend a lot of time blaming the atmospherics for poor seeing whereas a good proportion might be due to that layer of warmer air hugging the front of my 2" thick mirror. I guess I'll only know if I try it out rather than theorise the answer... Any practical experiences out there?

If I installed a pair of fans to blow across the front of the mirror, I'm wondering if I really need extra exit holes opposite the fans. My mirror sits in an open tailgate/mirror box design.

Thanks for your patience on what must be an old topic...

Chris

[DavidU]

Chris, this one way of doing it.
http://www.iceinspace.com.au/63-400-0-0-1-0.html
Although what Bird(Anthony Wesley) said is very true.

[Paul Haese]

Chris, this might appear to be an old topic but there is plenty of new thoughts on it. Fans for removal of boundary layer problems really is not addressing the real issue. Mirrors will release heat for hours and hours if no active cooling takes place. Fans at the rear of the mirror are certainly a step closer to getting the mirror to ambient, but there are faster methods. Peltier cooling takes care of this problem. It gets your mirror to ambient quickly and allows more time for viewing and imaging or whatever you prefer. It does require power which can be problematic but the results will be worth the effort.

[Insomniac]

I take your point, Paul, but surely while the mirror is slowly reaching ambient temperature it's better to disrupt that boundary layer rather than leave it hugging the glass?

[Paul Haese]

While I can see that using a fan at the rear is worthwhile I am yet to be convinced of fans blowing off the boundary layer. It is just adding more turbulence in front of the mirror and that has to affect the view. Each to their own though I suppose. :)