(ATM) Re:

[akelly@ghgcorp.com]

In reply to this post:
>
>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?
>

        It has been my experience that at least 1.5" should be allowed
beween the inside wall of a Newtonian's enclosed tube and the edge of the
primary in order to reduce the optical effect of tube currents.  In short,
if you are building a 12.5" scope with an enclosed tube, you should use a
15.5" i.d. tube.
This "rule" applies to scopes about 20" and smaller.  Larger, thicker
mirrors should require a bit more space.  Your tube should extend AT LEAST
twice the length of the diagonal's minor axis beyond the focuser.