From: clnoble{at}email.arizona.edu
To: Ross Sackett 
Cc: atm{at}shore.net
Reply-To: clnoble{at}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

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