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

["Arjan te Marvelde" <arjan.te.marvelde@hetnet.nl>]

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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