Re: [ATM] The Flexed mirror

[David Weinshenker <daze39@earthlink.net>]

Russell Jocoy wrote:
> As Bob May mentioned, and which was brought up on this list
> years ago, was to flex the polished, spherical mirror the
> opposite way and polish a "new sphere" on the surface, then
> "un-flex"  In theory you would have the classic Parob.. 
> Has anyone attempted this??

I've seen references to this as the process of
"stressed mirror polishing" in the construction
of large telescopes with large multi-segment mirrors:
for each segment, an oversize blank is mounted on a
fixture that clamps the edge down while supporting
the center, so that the blank is flexed the reverse
of the desired deformation from a sphere - then a
spherical shape is polished and the tension is released.

(With a finite number of edge clamps, the figure of the
overall disk had some distortions at the edge, but the
process was designed so that the intended figure would
be achieved over the surface of the hexagonal piece that
was cot out of the original blank for each segment.)

Bernard Schmidt's original correcting plate for a wide-field
telescope was made in a similar manner: he started with a
flat disk of glass, and mounted it on a fixture that supported
it around the edge while he drew a vacuum behind one side -
then he polished a flat surface on the exposed face, leaving
the center of the glass slightly thicker, so that when the
glass was removed from the vacuum fixture it had the desired
slight convexity.

-dave w



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