To: atm{at}shore.net
From: mdholm{at}telerama.com
Reply-To: mdholm{at}telerama.com


Hi,

I have been thinking about knock down "tube" designs.  The
"Alice" style with two parallel tubes supporting the upper cage
seems like the minimum for providing passive alignment on reassembly while
involving only simple hardware.
 I can imagine an "Alice" style scope using only one tube to
support the upper
works, but one degree of freedom in the alignment becomes unconstrained. 
That is the motion of rotation about the axis of the single
"truss" tube.  This would
manifest as decentering of the diagonal mirror.  Sufficiently clever
machine work at both ends might allow one to fix the tube and upper works
in rotation with the requisite precision, but getting axial alignment,
length repeatability and rigidity with a fairly simple design sounds rather
unlikely.  My thoughts are complicated by the fact that I want the
"truss" tube to break down into two,
or possibly three, parts for compact storage.  Simple screw connectors
would have too much uncertaintly in final axial alignment.  Again, the tube
connection
has to provide axial alignment, length repeatabilty, and rigidity.

The idea that popped into my head is this.  Don't try to passively restore axial
alignment each time the scope is reassembled.  Instead, mount a cheap laser
pointer, on the fixed portion of one end of the scope, and a target on the
other
end.  After assembling the scope, turn on the laser, and twist the
"truss" tube slightly in its mountings until the laser beam hits
an index on the target. With this strategy, one trades a few seconds of
adjustment during each setup for
the compactness and perhaps lightness of a one tube "truss" while
retaining a relatively simple mechanical design.

What do you all think?  What sort of tolerances would the system have to
hold? Can these tolerances be met with this idea?  What geometry of the
laser and target would provide the best sensitivity?  Would there be a
simple way to use a
mirror or two to fold the path and increase precision?

Is this a bright idea or a dim one?

Mark Holm
mdholm{at}telerama.com

Mark Holm
mdholm{at}telerama.com

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