Re: ATM 6" f8 travel scope

[John Swenson <johnswenson1@home.com>]

Conrad,
I'm building a 6" f8 scope that fits in briefcase, including the truss.
Its been an interesting chalange, I decided to use thin carbon fiber
tubes to make a truss similar to what is used in the new style folding
camping chairs. In order to keep the poles small enough to fit in the
briefcase I need two such trusses stacked on top of each other. I spent
some time analyzing the design and came to the conclusion that a 5 sided
truss is much stronger than the traditional 4 sided truss used in the
chairs so all my version have been 5 sided. 

I've tried many different approaches as to how to attach the ends of the
poles together, I thought I had it all worked out using small brass
barrel hinges, but it turns out there is not enough length on the hinge
to glue into the tube, the glue breaks under stress, I tried many
different types of glue but none worked. I almost had the whole thing
assembled and was moving it arround and the glue joints started breaking
and the whole thing fell apart, back to the drawing board!

I think I have it worked out now, the hinge will be a piece of all
thread rod which goes about an inch and a half into the end of the tube
(plenty of length to make a good glue joint), bent into an "L" shape,
which passes through a hole in a weirdly shapped machined nylon block, I
spent the last 5 days working out the exact shape and size of said
block  to make all the forces symetrical, this was one of the problems
of previous designs, assymetrical design can wind up with very high
forces on some of the connections. 

The secondary cage is made out of the same carbon fiber tubes glued
together to make a truss octagon. The intersection of each tube is
wrapped with carbon fivber tape and saturated with low viscosity epoxy,
it makes a very strong light weight joint. I don't have a focuser yet,
I'm probably going to get a commercial delrin helical because it only
weighs 4 oz. 

The bottom end is a standard low profile dob design, but the parts are
hinged so the whole thing folds up into a small package. The bottom
parts are mader out of 1/2" baltic birch plywood, I didn't have to worry
about keeping the weight down as low as possible so I used what I had on
hand. 

At this point I have the rocker box, mirror box, mirror cell and
altitude bearings done but not the base board. Most of the parts either
fold over each other or nest inside each other. 

Some day I'll get some pictures up on the web, right now I'm just trying
to get it built (and stay built!).

I'd be glad to help you with working out a design similar to this if you
are interested.

John S.