[ATM] Cutting the sonotube for a dobsonian

[Mark Holm]

A. Marc Passy wrote:
> I'm starting to look at building a dobsonian, and I've got a question. It 
> seems to me that one could put in substantial cutouts (expecially 
> longitudinally) before affecting the ridity of the tube. Has anyone cut out 
> any sections of the sonotube to make it lighter and to add ventilation? Any 
> experience with how much can be cut out before comprimising the rigidity of 
> the tube? 

I haven't actually tried it.  My guess is that Sonotube is just barely
stiff enough as is.  Cutting substantial holes in it is liable to result
in an unstable device.  A whole lot of the stiffness of a tube comes
from its circular cross section.  Interrupt that and stiffness falls off
rapidly.

> Could the tube be compressed along the longitudinal axis with string and 
> rings top & bottom restore some rigidity? (Unfortunately, my engineering 
> education was just enough to make me dangerous without actually letting me 
> know anything. ;)

Doesn't sound too plausible to me.

I'd say, if you are going to that level of trouble, you should start
looking at a more sophisticated material and structure such as good
quality plywood with aluminum tubes in one of a variety of configurations.

How big is this scope going to be?  At 8 or 10 inch, with a moderately
short focal length, solid sonotube is a great choice.  You will need
some holes for a fan and vent holes, but not for lightening.  If you
need collapsibility for transport, or are looking at something bigger,
then you need to think about other materials and structures.  Do you
have a copy of Kriege and Berry?  Have you seen Albert Highe's scopes?
http://pw2.netcom.com/~ahighe/astroequ.html I think Albert's scopes look
great in the 12 to 14 inch range and the K&B type for larger.

Mark Holm
mdholm at telerama.com