Re: [ATM] Re: Pre-stressed figuring?

["Bill Kelley" <flex@commspeed.net>]

Eyesky et al,

You deserve congatulations for having re-invented the center pull "flex" 
mirror. It was published in S&T June 1992 by me. It nets about a four times 
improvement over a sphere when properly tensioned. The concept was further 
refined by Howard L Moore, Alan Adler, and me. Alan published in Oct 2000 
S&T a finite element analysis of a superior version utilizing a distributed 
load to tension most of the back of the mirror inside of the edge support. 
Reversing the sign ( i.e. tension becomes push) will provide you with all of 
the forces and locations for any particular mirror made as you propose. The 
active optic approach that allows field tuning of the optic in its cell 
under tension is the method that has been adopted, and is now common with 
many references to it in the archives. Also  discussions of compressing and 
releasing.spheres after figuring may be found in the archives. With your 
scheme what you make is what you get, including astigmatism, as you noted. 
Alan patented his work, but it has not been adopted by commercial TM's 
despite the ease of producing ultra smooth spheres for flexing. There are 
some desireable applications for compression tensioning and releasing 
mirrors, such as convex secondaries for Cassegrains. I urge you not just to 
post to the ATM list. Make a flex mirror for yourself , get a good physical 
workout, have some fun, and if you make a fine sphere and mount it properly, 
you will have a splendid telescope.

In specula veritas,

Bill Kelley

You wrote-
>There seems to be little in the way of first-hand reports of grinding
a mirror while under tension (at least that I've found so far).  There
is a note that an application of a single point force to deform a
spherical surface would result in, at best, a 1/2 wave error at a
particular radial distance (if I understand correctly).  So forming a
true parabaloid would need application of force in a more complex
fashion than from a single threaded bolt or what-have-you.  It's still
unclear to me whether these results are 'good enough' for amateur
applications or not.
Also mentioned: complex equations for the forces needed and resultant
deformations, but I haven't actually seen them in print yet.<

>As for an experimental setup, I imagined epoxying a ring to the
mirrors edge.  Attach a flat plate to this ring, with a threaded hole
in the center.  A fine-threaded bolt (with suitible protection against
damaging the glass) could be tightened to apply a force.  I don't know
how unifrom the pull would be on the edge in practice, though.
It seems concievable that one could unwittingly grind in asygmatism or
other unwanted optical deformaties into the glass if you weren't
careful, so there are some unknowns still.

I won't likely be testing the process myself soon, regardless.<
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