Author Topic: Need advice on boundary layer fan for large dob  (Read 2210 times)

mccarthymark

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Need advice on boundary layer fan for large dob
« on: October 03, 2014, 10:47:36 AM »
For those of you who have an Obsession or other large dob with a mirror box, I need your advice for boundary layer fans.

Normally there is only a fan blowing from the bottom toward the bottom of the primary.  This seems ok for cooling but doesn't hit the surface of the mirror / boundary layer.

One alternative is the Teeter design, where they add two or more fans on the side of the mirror box to blow across the surface of the mirror, with exhaust holes on the other side.  This seems perfect -- except Nor. Cal. observing sites are typically in dirt parkland parking lots.  I suppose any dust would be blown across the mirror and not settle, but if I were at a multi-day event with lots of dust blowing around, even if I made the effort to cover the scope, it's likely to get dirty during the day.  I was at CalSTAR this year and the wind was pretty bad in the afternoon with dust devils swirling about.

Another alternative is to use a stiff plastic sheet of some kind, and use it to seal off the bottom of the mirror box with a cut out for the collimation knobs and the fan itself.  The fan would blow outward.  Seems this would be better for dust control, and if sealed properly give a good laminar air flow.  I could put a low CFM fan somewhere above and to the side of the primary to take care of the boundary layer -- or even suspend it over the center of the primary (though I'd have to remove it every time I needed to collimate).

Appreciate the advice & experience of the big dob owners out there.  I'm in process of restoring a 20" Obsession so have the opportunity to make improvements to it.

Thanks,
Mark 
Mark

John Pierce

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Re: Need advice on boundary layer fan for large dob
« Reply #1 on: October 03, 2014, 11:04:35 AM »
my seat-of-the-pants physics suggests that the air blown at the back of the mirror will spill around the edges of the mirror in turbulent eddies, and this likely will break up any static boundary layer.   when I was running a fan blowing at the back of the mirror on my 10" cardboard tube dob, it certainly seemed to help with the tube currents early in the evening where the warm air rising in the tube would be on the upper side... but my fan was in a completely closed back with only the fan intake, blowing at the back of a mirror that was sitting on 3 'standoffs' (blocks of wood about 1x1x2", which served to raise the focal point to compensate for the taller focuser I'd installed).