Re: [ATM] Strip planked telescope tube

["Bob May" <bobmay@nethere.com>]

Wood is a fairly low thermal cap. material thus it works well for
keeping currents inside the tube low.
I built my 200" refractor by the method of gluiing together
several strips of wood (cherry in my case) which I held together
while drying with a strip of rubber.
For the rubber, I found an old tire tube from a car (a truck tube
will also work fine - bicycle tires are way too small for the
amount of rubber strip that you want) and cut it into a strip
about 1" wide and many feet long.  Also cut some short pieces
that are about 1/2" wide for starter pieces.  You'll only need
two or three of them that go around the finished tube.
Start by cutting the pieces accurately to shape with the angles
of the joint between the two pieces of a joint.  For example, an
eight sided tube will have a cut angle of 22.5 degrees which,
when put together with another piece, makes an angle of 45
degrees.  Use the narrow strips of rubber to hold things together
and get used to doing the assembly fast as you need to go fast in
the next step.
Then comes the fast part where you slather all of the joints with
glue and then assemble the tube with the narrow strips of rubber.
When you get the tube sort of round (it should be round if you
cut the angles correctly!), you can start wrapping the wider
rubber around the tube.  Start in the middle and do a somewhat
loose wrap (don't try for much extension of the rubber) until you
get to one end and do the wrap at a fast spiral.  When you get to
the end of the tube, it is time to go back to the other end
wrapping very tightly and overlapping the rubber if possible.
Let the tube dry and unwrap and you've got a neat tube.
If you do a lot of pieces, it is always wise to make a disk or
two that will barely fit into the tube before you start gluing
and insert them into the end.  This will insure that the tube
stays close to round.  With 6-8 or so sides, the angle needed on
the glued edge is a lot easier to do and errors will show up with
a tube that rocks about.
Make sure that you provide extra length on the pieces so that you
can cut the tube to the right length after it is glued together -
you can't really get the pieces aligned in that direction well
enough that you don't need to trim them.
Bob May
bobmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
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