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Don't mean to knit-pick here...

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Glad you did! I've been wondering about this issue myself. For a glass mirror that's typically *warmer* than the ambient air temp until it's acclimated, it's commonly portrayed as having this "boundary layer" sitting on its face, like a puddle. Yet in point of fact, the air absorbing heat from the glass will itself become warmer than ambient, and so will rise- across the face of the mirror, and thence up the tube- and thereby produce the well known & documented "chimney effect" common in tubed Newtonians. Check these out!

Yes, unless your scope's name is "Hubble", there will *always* be air directly in front of its mirror. At stake is what is that air doing? The plan is to keep it still, which a fan helps by eventually reducing the temp delta, and so the unwanted thermal effects. It will also, if blowing *UP* the tube (assuming we're aimed upwardly & viewing the sky ), scoot the temperature-laden "bad air" out the top of the tube, and much more quickly than if left to natural convection alone. Blowing up the tube works WITH convection, not against it. I believe it also smooths this air out, so it's not bubbling & billowing as it makes its way up the tube walls toward the top... which helps keep these effects farther out of the light's path, both down & back up to the secondary.

So as i see it, there are three possible states for this boundary *and* tube air, affecting light on both it's way down to the primary, as well as back up:
1) loaded with thermal energy- turbulent, churning, billowing all over the place, or
2) loaded with thermal energy- but gradually decreasing, and being whisked into a smoother, directional flow outta there, or
3) ambient- and relatively quiescent.

It's pretty easy to see the diff between 1 & 2, by switching a Newt's rear-blower fan on-and-off while viewing;
*ON* definitely & obviously improves the views!

Once you're at 3, can the fan, man, and run up the mags until the sky yells "uncle!"

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Don't mean to knit-pick here but there is no removing a fluid boundary layer.

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Hi Sixela,

I just read your 2010 post. Do you recommend to use the fan the other way round to pull air?

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Hi everyone. Thought I'd chime in here with a related question. I saw a similar discussion earlier this year (on this CN forum I think) in which fans were mentioned that have a very good score for minimum vibration, and I wonder if anyone knows what make those are or can recall that post. I was pretty new to CN then and forgot to bookmark it.

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If it is strong enough. Most computer fans are not.

I bought an "outrunner" motor used for radio-control airplanes and
mounted it behind my mirror cell with a 9" APC pusher prop. I run it at
10000 RPM. Gets rid of every trace of tube currents and boundary layer.
The only problem is you have to be careful getting your hand close to it,
because the prop can take a nice chunk out of your finger. Collimate
*before* turning on the fan.

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... I saw a similar discussion earlier this year (on this CN forum I think) in which fans were mentioned that have a very good score for minimum vibration, and I wonder if anyone knows what make those are.....

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