Re: ATM 3/4" plywood for dob.?

["Herb Kasler" <h.kasler@worldnet.att.net>]

Mike,

I've been thinking about this problem also or my first scope, a 16" f/4 dob.
I too was thinking 3/4" ply at the outset, but was told it would be way too
heavy. A few quick calculations bore out the truth of this admonition. I'm
currently making the optical assembly out of 1"X1/8" and 3/4"X1/16" square
cross-section aluminum tubing. I'm brazing it together with some stuff
called Alladdin 3-in-1 and popriveting on gussets of 1"X1/8" strip at the
major stress points. The brazing took some practice before I could get a
really good joint.  The whole tube will weigh about 20 lbs. minus the 22 lb.
mirror when complete. The gussetted brazed joints seem very strong indeed in
the mirror box, which is all I've finished building so far.  I can sit or
stand on it no problem.  I haven't yet tried jumping up and down on it. I'm
thinking for the base that I'll also build a frame out of this material or
out of 1X2" wood and just face it with very thin sheets  of plywood screwed
on where needed or desired. The bearing cradle could be made out of 3-4
segments of 2X4 or even 1X4  mitered together into a partial hexagon or
octagon and then cut to a circle on the inside. this would just be held up
by an a-framelike structure of wooden or aluminum members, cantilevered at
the base onto the circular bearing platform. Of course for a smaller scope
(you don't specify the size of yours) you could use smaller material. Any
facing, except on the azimuth bearing surfaces, would be purely decorative.
This will yield a much lighter structure than anything made out of solid
sheets of plywood. Certainly, for a bigger scope like mine, 3/4" ply would
just be ridiculously heavy, like 130 lbs. just for the mount. Ugh!

Good luck,
Herb.