Re: [ATM] please help with mirror cell design

["Bob May" <rmay@nethere.com>]

One of the few thing about telescope design that really hasn't
been talked about that much is the designing of the mechanics of
many pointed trees.  Until just recently (maybe twenty years)
thin glass reeally wasn't used for mirrors  and thin mirror
support reeally wasn't needed.  Today, with the advent of the
Internet,  books really aren't used as much to expound on ideas
so your best is to go look at how others have done their mirror
suppor designs and maybe ask them to take some more detailed
photos of their supoort system.  There is the program PLOP which
is freeware from one of the guys on the list that really puts the
points in the exact proper place to give the best support for the
thin glass so that the error of the surface is minimized.  The
PLOP program uses a basic version of Finite Element Anylisis to
figure out where the best places for the supports are to be and
sometimes it comes out withy some supprising results as on of its
features is that it looks for keeping a parabola even if that
means the glass is bending in the process
One of the little supprises that came out of that program is that
the "traditional" meethod of supprting a mirror really isn't
right, that being support at the seventy percent zone but rather
that the support should be at about the fifty percent zone which
turns the mirrro into a slightly longer focal length parabola!
The flexing ability of a point on the tree doesn't need to be
much - a degree of motion is more than sufficient but it does
need to be fairly easyt to move that little bit  otherwise the
force of bending that point will be reflected up to the mirror
surface and PLOP depends upon that resting point of the point to
be with zero forces on the glass to hold it in place.  I'll also
note that the position of the points is only approximate in that
a 1/16" error won't hurt anything so you don't need a real
precision machine shop to do the work.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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