Re: [ATM] Gimbaled Secondary mirror

["matt" <electro_optic@bellsouth.net>]

I believe the original question was probably meant to ask why 3 points
equally spaced at 120 degrees from each other, rather than a center pivot
and 2 points spaced at 90 deg , leading to independent x/y tip/tilt . The
arrangement that is prevalent now, with the 120 degree spaced 3 points, has
interdependent motion , you adjust one point and the other 2 need adjusting
.
The 3 points arrangement with one central point and 2 others at 90 deg from
each other and equally spaced from the central point do not have this
problem.

best regards,
matt tudor

-----Original Message-----
From: Bob May <bobmay@nethere.com>
To: atm@atmlist.net <atm@atmlist.net>
Date: Friday, July 01, 2005 2:31 PM
Subject: Re: [ATM] Gimbaled Secondary mirror


>The reason for a 3 point mount is easy.
>Three points define a plane.  This plane can be moved about without any
loss
>of accuracy or "looseness" in the system and doesn't require the
>compensation with other points to do a move.
>For example, the next number requires the push by one actuator and the
exact
>same pull by the other actuator to do a move.  If you fail to provide the
>exact opposite motion, the surface is stressed by the motion being
different
>between the two actuators and this ends up with an incorrect motion at the
>best and a deformed mirror at the worst.  You can compensate for this a bit
>by putting springs on the opposite of two actuators but if you don't
provide
>a center point (making it a 5 point mount) you then end up with complex
>motion calculations to do the job.  In addition piston motions become a bit
>more difficult.
>Bob May
>bobmay@nethere.com
>http://nav.to/bobmay
>http://bobmay.astronomy.net
>
>_______________________________________________
>ATM mailing list http://www.atmlist.net/
>

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