Re: [ATM] trilateral scope problem

[Mike Lockwood <melockwo@uiuc.edu>]

Jan,

Jan van Gastel wrote:
> The problem now, is that the scope doesn't hold collimation because of
> bending. From about 25 degrees altitude to almost vertical, there's a
> difference of about 2-2.5 millimeters. As far as I can see, only the two
> front tubes are bending, which makes sence I think, because they take the
> compression load. The only solution that might work is placing the two front
> tubes on the inside of the side bearings (which make them shorter), and/or
> use larger (in diameter) tubes on the front side. Outside diameter of all
> tubes is 22 mm and 1.5 mm wall thickness. Lenght is about 2345 mm (measures
> in inches: 0.865 tube outside diameter, 0.059 wall thicknessand 92 inches
> length). But possibly there is no simple solution (although I still hope
> there is one).
> If anyone has ideas, I would be glad to hear them.

I think you're on the right track if you are considering larger tubes. 
  Of course they will throw your balance point off.  :(  This is a 20" 
scope, correct?

My main bit of advice is to be sure that the collimation shift is due 
to flexing tubes and not something else.  Lots of other things can 
cause the drift, especially slop or flex in a mirror cell, slop at the 
pole attachment points, or a flexing secondary cage.  (I'm sure you're 
familiar with those problems, but I want to point them out to others.)

Due to the nature of a truss, though, I doubt it's only two poles 
bending - they probably all are to some extent, or their attachment 
points are not perfectly rigid (or both).

To be sure the problem is what you think, you could temporarily attach 
(tape) some stiffer poles/beams to the truss poles to reduce flexure. 
  If the problem is markedly reduced by that, you have pinpointed one 
cause.  My suspicion is that you may find that ALL the tubes need to 
be a bit larger.

Another illustrative test is to clamp the mirror box firmly to 
something very solid.  Then grab the secondary cage/ring and pull to 
see what motion you can cause.  This will quickly lead to insights 
into how to reduce the flexure.

	Mike Lockwood

PS:  Are you using that 12" mirror yet?



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