(ATM) tube currents

[grant@aretha.llnl.gov (Chuck Grant)]

> From: Heiny Ed <heinye@rnd2.indy.tce.com>
>
> Recent posts concerning tube currents vs. tube diameter have me worried. 
>  I'm working on my own 12.5" f6 and helping with our clubs 12.5" f5 
> newtonian.  I've got a fiberglass tube with a 14" ID for the f6, and we were 
> planning on using a 14" sonotube for the f5.  Are these tubes too small in 
> diameter?  Also, what impact does the tube length have on tube currents, 
> i.e. how much beyond the focuser should the tube extend?  What about the 
> mirror end?  Does a longer tube reduce the impact of heat radiation from the 
> observer?
> 

What to do to most effectively reduce tube currents is not a question
that is likely to be answered the same way by any two people on this
list.

I think that 14" I.D. for a 12.5" scope is a little too small if I am
in the design stage. I like to have an inch of clearance all around
the light path.  But if I had a 14" tube already, I probably could
live with it.

Extending the tube farther beyond the focuser helps. It reduces the
effect of the observer's heat (it reduces the effect of the
convection from the observer, not the radiation).  The air nearest
the ground is the most turbulent.  Making the tube a foot longer so
you have to look through a foot less of this worst section of air
also can be significant.

Extending the tube very far beyond the primary mirror is usually not
done because it makes mounting the telescope much harder.  Except for
the smallest scopes, the mirror is always put as low as possible, so
everything else can be as low as possible.  Closing the tube, once
the mirror is in thermal equalibrium, is an option, as is an active
ventilation system.  This will help when there is a significant
temperature difference between the top and bottom of the tube.

Insulating the inside of the tube can help too.

Chuck