Re: [ATM] Use of bearings in (static) mirror cells

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

Hello list,
It's not just the cell parts that have to be balanced or hinged at their
COG's . The mirror itself and therefore its COG is hanging some distance in
front of the mirror cell, therefore creating a non zero torque. That torque
is what warps the mirror .

There are only 2 ways to eliminate it,

1-don't allow rotation and allow only longitudinal translation at the pivot
points, easier said than done, because it would require again flexure stages
.This would cure the symptoms without curing the disease .

2-don't allow the assembly COG to be in front of the hinges plane, which is
what creates the torque to begin with. This would solve the cause of the
problem , and it would require counterweights behind the hinges plane . This
is exactly what an astatic cell does .

best regards,
matt tudor


-----Original Message-----
From: Jerry <wa4guu@verizon.net>
To: 'Arjan te Marvelde' <arjan.te.marvelde@hetnet.nl>; atm@atmlist.net
<atm@atmlist.net>
Date: Friday, April 22, 2005 10:54 PM
Subject: RE: [ATM] Use of bearings in (static) mirror cells


Hello Arjan,


Referring to http://home.hetnet.nl/~artm/atm/articles/deformation.html

This raises several questions in my mind. I will ask the first as the others
dependent on my understanding of the answer.


Why assume that the cantilever as viewed in the vertical position (as in
Fig. 1B) is asymmetric in mass to the left and right of the fulcrum? Can't
we balance that so that it imparts no force to the glass that is a result of
its attitude from vertical all the way to horizontal?
Instead of shaping it like  ]  shape it like  I  .
If Balanced I do not see how it imparts any deformation to the mirror as a
result of tilt and the fulcrum being some distance from the glass.


Jerry



-----Original Message-----
From: Arjan te Marvelde

Hello Jerry,


These numbers can be compared with an analysis of deformation of a mirror
glued to its cell: http://home.hetnet.nl/~artm/atm/articles/deformation.html
As you will see, the force couple caused by glueing the mirror to the cell
is way more than this 3.6 gr caused by stiction:
120gr when pointing at horizon,
60gr when pointing at 60deg elevation,
0gr when pointing at zenith.




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