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

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

Hello Arjan,

In my opinion, you have to qualify your statement carefully. Your analysis
is true for certain  types of mirror cell configurations and not true for
others . You are not defining your configuration or boundary conditions so
to speak, which might lead people to believe the analysis is absolute and
true for all types of mirror cells.

Here are some examples where the analysis you did is not applicable.

Assume the support triangles were for example properly supported by flexural
stages that only allowed translation along the mirror optical axis and no
rotation (only piston movement for supports) .
 You are analysing a torque effect (due to mirror COG being in front of
support pivots, and supports being allowed extra degrees of freedom
including rotation around pivot) that induces astigmatism obviously. If the
supports were restricted to strictly translation instead of allowing
unnecessary rotation, the problem disappears. In that case, gluing would
actually be recommended, because it would eliminate mirror shift , and
astigmatism due to improper edge support.

Also consider another case, where the cell has astatic levers . Sum of
moments around the hinge is null and there is no resultant torque to induce
astigmatism (although the supports might be allowed to rotate ).

The obvious conclusion being that before deciding for or against a certain
method , the initial assumptions of the analysis must be carefully stated
and analysed too , otherwise it's GIGO . People tend to believe what they
see coming out of computer CAD analyses as absolute .

And last but not least, mu remarks were not meant to detract anything from
the value of your analysis, just to point out some cases when it's
applicable and when it's not .

best regards,
matt tudor

-----Original Message-----
From: Arjan te Marvelde <arjan.te.marvelde@hetnet.nl>
To: atm@atmlist.net <atm@atmlist.net>
Date: Friday, April 22, 2005 10:14 AM
Subject: RE: [ATM] Use of bearings in (static) mirror cells


>Hello Jerry,
>
>In my opinion, flexures are OK as are the RTV flexures, as long as you
don't
>glue them to the mirror.
>No one seems to care though.
>
>Stiction is no big problem if you choose the proper materials.
>As an example, PTFE (Teflon) on steel has a coefficient of kinetic friction
>of only 0.05 or so, and the static cof is only slightly more, probably 0.06
>or so. Consider a lever in a cell of 75mm long: the force you have to
excert
>to overcome stiction is in the order of 3.6 gr, assuming a bearing load of
>600gr and a working diameter of 7.5mm  (which I consider pretty large).
>Smaller bearings will have smaller effect.
>
>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.
>
>My conclusion is that you will only see the effects of stiction when the
>mirror slides on its support points. You do need a side support, but the
>deformation caused by such (properly built) side support is close to none.
>
>Cheers,
>Arjan
>
>> I have noticed Don's zeal urging consideration of the compliant
>> hinges and/or flex bearings for various telescope parts. This discussion
>has
>> been helpful for me to understand his zeal. I'd bet his would be a very
>good
>> mirror cell. I'm surprised there is so much resistance to his
suggestions.
>> But then I am not an enginier.
>
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>

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