FWD: Re: [ATM] fans

["john sherman" <atm@mail.johnspics.com>]

Hi there, 

>How do you know that you were homogenizing the 
>boundary layer in front?  

If you point your scope at a bright star and rack an eyepiece way out of focus (beyond star-test range), you can learn a lot about the inside of your scope. For example, collimation problems can be seen, bent spider vanes, etc. But best of all is that it allows you to see the turbulence in the light path. 

There can be a variety of things contributing to what you see. Usually a high-speed flicker is the jet-stream or other high-altitude currents. Medium-speed fluctuations are often within 1000' of the ground (plumes from houses, etc). If you have a warm-mirrored Newt with no fans running, you should plainly see a plume of air rising off of your mirror (*up* in the eyepiece is probably not *up* in the scope). Now you can experiment with fans, turn one on and see what effect it has. Can you destroy the plume in front of the mirror by using a fan in back of the mirror? I do. 

You can blow a fan across the front of the mirror and see the plume of air completely chewed up into very small, constantly moving cells. The damage from the heat is no longer acting like a single, slowly-moving  lens, it is now thousands of tiny lenses affecting the entire aperture evenly. But it is still damage. The purpose of a fan is to get your mirror cool, then turn the fan off.

Yes, it is possible that there is a residual small bit of the boundary layer left, hidden behind the rapidly moving small cells. But I've never seen it, so it would have to be a very small bit, and only when the mirror is still too warm to observe through anyway.

Best to do this testing when the seeing is stable, not on one of those nights when the stars twinkle so violently that they're threatening to fall out of the sky. That way there are less issues to confuse things. 

So when you get your mirror cooled off the entire scope should be at equilibrium (or close enough for our purposes). The closer it gets, the smaller the plume of air rising (with fans off). Eventually there will be only a tiny wisp of hot air rising off of the mirror. If you get your scope to equilibrium and the fans are off, then the only turbulence you should see is outside of the telescope. This is a perfect time to ask someone to stand in front of your scope, so you can watch his plume destroy the image. 

It is very easy to see and experiment with the turbulence in the light path. I expect y'all will come to the same conclusions I have.

Later,

John





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