From: Mark Holm 
To: atm{at}shore.net
Reply-To: Mark Holm 


Hi ATM's

I have, I hope, finished figuring on a 205mm f/5.84 mirror.  Now I have to
plan and build a telescope around it.  I want to make a collapsible
structure that will knock down to carry-on dimensions.  I am not sure about
design.  Something like Paul Lefevre's 6-inch or Gary Seronik's 8-inch was
my first inclination. Gary gets by with 4 "truss" tubes since his
focal length is short (f/4).  A while back, I used the formula in Kriege
& Berry to calculate that if I increased the tube diameter from 1/2
inch to 3/4 inch, I should get equivalent stiffness despite the longer
length.

Paul's and Gary's designs are safe in that they have been built and shown
to work reasonably well.  I have been contemplating a slightly more radical
design.
  Instead of a full upper cage, cut the upper structure down to just enough to
hold a focuser, a 1 or 2 strut "spider" and a small reflex finder
(probably the Rigel unit).  Then, cut the "truss" down to 3
tubes.

The three tubes would be arranged in an off center tetrahedron.  Assuming a
square mirror box, number the corners 1,2,3 and 4.  One tube would arise
parallel to the mirror axis from corner 1.  The other two would rise from
corners 2 and 4 and angle to join tube 1 near the focusser.  The
tetrahedron would actually be truncated at the top.  The tubes would not
meet, but would terminate at a clamping assembly that would also serve as
the structural link to
the focuser/finder/spider unit.  This is somewhat like Mel Bartels' little 4.25"
except that his has two parallel tubes plus the two angling tubes.

A possible problem has entered my mind and I would like to have others
thoughts on it.  It seems to me that the truncated tetrahedon truss is not
inherently stiff in torsion about an axis parallel to (but offset from) the
mirror's optical axis.  Am I likely to get vibration troubles because of
this?

Has anybody tried a similar design?  How did it work out?



A partial reason for wanting to minimize the number of truss tubes is that
with my longer focal length, I will probably have to segment the tubes to
cut them down to carry on length.  That will need machined connectors and I
want to keep the amount of machining small.  (I may be able to sweet talk
my Dad into turning
them on his lathe.)



An even more radical design, though structurally simpler, would be to use
exactly one, larger diameter "truss" tube, perhaps 50 mm or so in
diameter. The
problem I see with this idea is twofold: 1. A good way to break the tube in half
and reconnect it securely, coupled with 2. A good way to align the tube
halves and the tube base so that alignment is maintained when they are
reassembled. The part that troubles me is how to get alignment in the
rotation axis about the
tube's long axis. If not for the need of rotational alignment, one could
just use some form of screw coupling.

So, does anybody have any good ideas on how one could join and align a
single tube "truss"?

Thanks, and happy New Year.

Mark Holm
mdholm{at}telerama.com

--- BBBS/NT v4.00 MP