From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "Mark Holm" 
>
>Consider very strongly putting in a fan blowing across the face of the
mirror,
>with vent holes or a suction fan opposite (if the tube isn't open).  Very
good
>evidence presented in two Sky & Telescope articles shows this will almost
>certainly help get the best out of this mirror.  A light metal tube will
>probably have fairly high resonant frequencies and low damping.  This makes
it a
>bit more likely that vibration from the fan(s) will be a problem, but there
are
>ways to deal with that.  (I think I read on this list that plain bearing
>computer fans run smoother than the ball bearing ones.  Panasonic, I think,
>advertises computer cooling fans with a special plain bearing design they
call
>"HydroWave".  Supposed to be very quiet (smooth).)
>
>Mark Holm
>mdholm{at}telerama.com

Hi
 One problem with most computer fans is that they tend to
cause the air flow coming out to be rotating. This cases an interesting
effect when the column hits a flat surface. It will tend to spread to the
sides with almost no flow in the center. While this may be what you want it
surely isn't what you'd expect. A better method might be to use some of the
flexy tube to run the air from a remote fan, built into the base( more
coupled to the ground ). Then, you could use a spreader to get a more
uniform laminar flow across the face of the mirror. Dwight

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