Author Topic: Plan:Boundary fan suspended over 16" mirror.  (Read 34 times)

Rick Perrigo

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #15 on: January 31, 2018, 12:19:26 AM »
Yes you can make a fan suck. But its not an improvement and it won't help the boundary layer.

Isaac Griffin

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #16 on: February 02, 2018, 03:56:06 PM »
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Yes you can make a fan suck. But its not an improvement and it won't help the boundary layer.


It does if you fit the aluminium plate to create a rear seal.

Steve Lewis

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #17 on: February 09, 2018, 03:00:50 AM »
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So thats my plan! Its an OOUK 16" f4 with a tube thats not much larger than the mirror. Im convinced stirring the air up will help hugely - my boundary layer is certainly an issue. My impression is this does a better overall job than perhaps 2 fans in holes in the tube wall. Its also less damaging to the structure should I wish to get rid of it!

I've seen examples online but cant seem to find any now so would appreciate any advice or experience, in terms of positioning and suspension methods especially. And recommended cfm.

I'm planning on using something like a 50mm fan (so ~70mm diagonal - theres a 100mm secondary so plenty of shadow to sit in) suspended something like 6-8" above the mirror, with four thin wires, two of which would carry power. Zero chance of aligning perfectly with the spider but I think the extra diffraction is a price worth paying.

Please help!

At f/4, though, off-axis rays hit some of the shadowed primary. So a secondary has an umbra (permanent shadow) and a penumbra (partial shadow).
To guarantee the fan doesn't block the light, it will need to be 1/2 or less the diameter of the secondary mirror and exactly centered under the secondary.
If you use a centered secondary, after collimation your optical centerline will be tipped toward the focuser slightly, which means the umbral shadow of the secondary
will not be exactly centered on the primary mirror. Likewise, your fan will have to be off-center away from the focuser by a very small amount.

It will be far easier for you to put holes in the tube to both blow air on the primary and evacuate it on the other side.
But if you insist on using a fan suspended above the primary, make sure no dimension (including the diagonal if it's square) exceeds 50mm.

maogrinjorli

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #18 on: February 09, 2018, 11:08:21 AM »
I dunno if this is relevant or not, but it's related I guess...This guy has several demonstration videos of this tube current stuff..

https://www.youtube....h?v=UbH4AS-0ViE