Re: ATM 2 Piece Poles - and how trusses really work

[clnoble@email.arizona.edu]

Hello everybody,

I have a few comments in regards to the "compression arch" string.  I recently
had the joy (nightmare) of building a travelscope in which I built a
compression arch for it with four poles.  Although my poles are one piece and
not two, I think that some of this may be relevant.  Just some ideas.

Mirror Box Stiffness-  Mirror Box stiffness is critical because if the box
flexes, your collimation could change.  In order to stiffen mine up, I attached
four pieces of plywood with 45 degree angles on the inside corners of the
mirror box.  Not only does this significantly increase the stability of the
box, but it also provides for a nifty place for your poles to slip into,
without having to secure them with a small bolt.

Box Joints- This is the most critical part of the design.  I first experimented
with using bolts to secure them into the box, but the poles moved around too
much. Instead I slip the poles into the corners with those 4 plywood pieces. 
It fits with friction.  This allows for more contact between the pole and the
mirror box resulting in less unwanted movement of the pole.  This means that
when I take the telescope apart, it's collimation is only off a little bit.  If
I were going to rebuild my telescope, I would make sure that those plywood
pieces making up the corners would be longer than the 5 inches that I made
them.  The more contact that you have with the mirror box the better.

A quick question.  When you guys calculate the compression and tension of these
poles using Young's Modulus and the cross sectional area, do you account for
the tube being hollow?  I know that the equation can be used to figure out
compression and tension on a solid item, but I didn't know it could be used on
a hollow one.  Do you just use a different value for Young's Modulus if it is
hollow, or are you just using the modulus for aluminum?  Or is everything the
same.

Thanks,
Colton