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Re: [ATM] PLOP Calculations
Andy,
> The 37" will be collimated by tilting the entire primary complete
> with its cell so maintaining the location of the sling should
> not be a problem.
That should simplify the adjustment a lot!
>
> >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).
>
> I'm a little leery of letting the sling find its own preferred
> position..I'd at least have non-contacting guides to limit the
> excursion.
Perhaps if you want to clamp (or tape!) the sling, the best position to do
so would be at +-45 deg from "down"
>
> >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 agree. Some of these problems have already been solved by the
> pros, like mercury or air bag edge flotation, using a cetral perforation
> for the edge support (done this & it works), etc.
New mercury bags won't be made today I am afraid....
But judging from the pros' work, the simple-to-make but theoretically
optimal solutions seems to be the 180 deg sling, or for the simplest one,
+-45 deg support points (using Géa's piano wires, or ball bearings as I
like!), with the simple astigmatism cancelled out (even if other errors,
like coma, are not).
Come to think of it, the edge supports can cause deformation in 2
essentially independent ways: one is by forces entirely perpendicular to the
optical axis, where pressure at the edge will squeeze the glass thicker near
the edge, causing the glass to bulge out here - this I am sure is the least
problem for amateur sized mirrors, and besides it should be worse with
*thicker* mirrors. The other way is by forces that have components parallel
to the optical axis - applied at the ends of one diameter, even a quite
small fraction of the force on one back support point can totally unbalance
the whole setup and cause serious astigmatism - this is what I believe is
the critical thing to avoid.
Nils Olof
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