Re: [ATM] PLOP Calculations

[Andy Saulietis <iss@pvtnetworks.net>]

Hello Nils,


>Perhaps if you want to clamp (or tape!) the sling, the best position to do
>so would be at  +-45 deg from "down"

Makes sense...

>New mercury bags won't be made today I am afraid....

Not to mention how much the stuff costs..

Water bags etc also come to mind but create
another problem - large thermal mass near the
mirror..it's hard enough to equalize a big mirror
already.


>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).

Richard suggests the RTV blobs instead of low friction
axial supports, dispensing with the sling entirely.
My intuitive take on this is that attaching anything to
the mirror not having the same CTE is asking for trouble.


>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.

I agree with that analysis..it boils down to making a sling that
does not impart any forces on the mirror other than the weight
of the mirror, ie the force on the sling should be along the radius
of the mirror in the plane of the mirror. Ideally this force would
be smoothly distributed, like a Plop cell with an infinite number of 
support points.
Making the sling/mirror contact frictionless would be the ideal
solution, but a finite number of low friction support points like
Teflon beads strung on a cable would be more doable.

We'll know soon enough after I build the cell..:)


Andy Saulietis
ISS Enterprises
10 Milky Way
Mayhill NM 88339


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