From: "Tom Krajci" 
To: 
Reply-To: "Tom Krajci" 


>From: Ross Sackett 

>Within their range of motion wobbling pivots like ball
>joints don't care about their orientation--you move
>them a little and they stay put at their new angle.
>This seems to me what we want in mirror cells...

As long as stiction and slop don't spoil things.  In smaller/thicker
mirrors this is not a problem.  In larger/thinner mirrors this may become
an important consideration.

>But compliant joints don't work this way.  Compliant
>joints are like springs--displace them a little, and a
>restoring force acts to bring them back to their
>original position....

Yes, that's correct.

>...If everything in our telescopes
>was made perfectly square and plumb this wouldn't be a
>problem--the telescope would be perfectly collimated
>at the neutral position of all the joints and each
>support point would carry exactly its intended
>proportion of mirror weight.

Again, sounds like correct thinking here.

>But real scopes aren't perfectly square, plumb, and
>on-axis. When we twist the collimation bolts to align
>a cell made with compliant parts, the pivots between
>triangles, bars, etc., must rotate a bit, bringing the
>restoring force into play.  This changes the
>distribution of forces on the mirror--some support
>points will push less, while others push harder as the
>compliant joints struggle to regain their neutral
>positions.  Inevitably the mirror will flex in ways we
>don't intend, and we get astigmatism.

>...don't compliant joints inevitably change the
>force distribution on the mirror supports when we
>adjust the cell?

Yes, the spring/restoring force comes into play.  As I think more about
complaint cells for mirrors, it makes sense that you must take care to make
your tube and cell as square/straight/etc. as possible.  Also, you can
incorporate the possibility to initially install/shim/adjust your mirror to
approximate collimation *without* adjusting the compliant hinges...and once
you get close with this initial installation...then you can use the
compliant hinges to make final collimation adjustments.

One nice thing about Richard's approach (RTV fabricated as the compliant
hinge) is that you automatically make the hinges in the zero stress state. 
Then you can install the mirror cell without touching the collimation
adjustments...and adjust your installation (shims?  Other?) to get
approximate collimation.  After that point you can use the compliant hinges
to make final, small adjustments.  That will keep spring/restoring forces
low.  How low?  I can't be specific at this time.

>I would imagine that this systematic
>effect would be quantitatively far worse than the more
>random (and I think small) effects of slop and
>stiction in wobbling pivots.

Lots of variables at play here.  Depending on the geometry and type of
material used in the compliant hinge...it may not be very stiff, so that
restoring force does not get very high unless you are very sloppy in your
initial construction/installation of mirror and cell, and then must use a
large amount of adjustment of the compliant hinge to achieve collimation.

However, this is qualitative.  We need more specifics here to see how much
of a problem this may be with compliant hinges.

Thanks for raising a good point.

Tom Krajci
Tashkent, Uzbekistan


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