(ATM) Re:

[jbmcmath@intellinet.com (bruce mcmath)]

>
>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?
>
I'm concerned about this myself, and want to take advantage of your inquiry
to ask how effective insulating material is in allowing a reduction in the
2" gap recomended by Texarau.  I will say that there seems a consensus that
aluminum tube is the way to go not only for reasons of thermal issues but
flexion and expansion etc.