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Re: [ATM] PLOP Calculations
Andy,
> I thought the sling has to be in the plane of the CG of
> the mirror perpendicular to to optical axis..allowing the
> sling to move in that direction (along OA) would
> not be desirable?
You have a point there, but here's how I think about it:
If you could position the sling at the right level (of the COG) and
accurately parallel to the mirror back, it would be ideal. But if you can't
exactly (and I believe most mirror boxes won't let you do it easily), you
could either:
1) position the cable accurately at the mirror (by clamp or friction) and
have the cable/sling leave at some small angle, pulling the mirror edge
forward/back *or*
2) letting the cable (with bearings) roll against the mirror edge and find
its own best position - perhaps a little away from the exact level of the
COG, but without any grip on the mirror edge.
I believe 1) is much more at risk for astigmatism, which is why I prefer to
keep close to alternative 2).
>
>
> >Perhaps a (moderately but sufficiently heavy) sling of steel wire (easier
> >than a chain), strung with small ball bearings at suitable distances
(with
> >spacers in between) would do the trick - suspended to touch the mirror at
> >the level of its COG.
>
> I used a 5/16" plastic coated steel cable for the sling on the
> 36"f5.2 2" thick currently nearing completion..showed little
> if any astigmatism. Another thought is to use small Teflon
> disks strung thru the cable, maybe with a thin plastic band
> around the edge to reduce the friction with a ground pyrex surface.
> Something like this for the primary rear support points might be
> easier than the radial bearings.
The whole problem of edge support has received less attention than the back
support, unfairly I think, but there is a wide open field for designing and
experimenting.
I have just made a quick-and-dirty webpage about my ideas for a 3-part
sling - see:
http://w1.411.telia.com/~u41105032/sling/threepart.htm
Nils Olof
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