Re: [ATM] Crazy simple mirror cell idea

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

Lets start with the misconception that larger mirrors need mour
support points.
The need for more support points is a function of the thickness
to diameter ratio.  At around 12:1 ratio, the mirror starts
becoming glexible enough to where the error budget starts
affecting the real shape of the mirror to the point where the
mirror isn't working right.  It's all in how much flex will you
allow.
Note that I didn't mention any particular size when I mentioned
the ratio?  That is because the ratio sets the amount of flex in
the structure, not the actual size of the structure for anything
of a real size that we'd make.
Next, you really don't want to hold a mirror at the edges by
compression as this will deform the mirror a lot more than the
deformation that gravity would do to the mirror.  Worse yet is
that the deformation would be random as there would be a random
amount of pressure on the edge of the mirror - no matter how
accurately you machine a mirror cell of this kind, it won't be
perfectly even in it's shape and thus, even with the evening out
of the rubber (until you get to the point where the mirror floats
about in the rubber) there will be an uneven push on the glass
edge.
For your mirror, three points at about the fifty percent zone
will be quite good enough that you will not be able to see in
testing whether the mirror is standing or laying down.

Bob May
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