Re: [ATM] PlOp PlOp Fizz Fizz oh what a relief itisn't? Mirror cell for an 8" mirror

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

IN other words, the equal area 9really should be equ8al volumne)
method of holding a piece of glass makes the assumptionn that the
segment is unsupported at the edges of that segment.  This is the
assumption that the old-timers made when thay set the points for
their mirror cells.  They corrected their bad assump6tion by
making the glass thick enough so that it really didn't matter
where you supported the glass.
\With today's desire to build lighter weight telescopes, the old
6:1 rule has been thrown out and much thinner mirrors are being
built which require better supports to keep the glass in the
right shape.
With the old equal area rule, you end up with the center and
edges unsupported and when the glass bends under its own weight,
the glass deforms to another shape.  PLOP does FEA in an
approximate way and puts the support points at a location where
the slumping of the glass under gravity forms a parabola, albeit
one of a slightly different focal length, and thus produces a
better figure.
Please note that we are generally talking of very small fractions
of a wavelengthrather than anything large and these variations
are in the region where you have to be very exact and proper in
obtaining interferograms to even see to begin with.  A mirror
built to a tenth wave accuracy is way beyond the errors that are
induced by bad locatoon of the mounting points (note that I'm not
even talking about stresses that may be induced by a bad
mo99unting!0 and would be rather difficult to see in practice.
This kind of reminds me of the old engineer that worked at KDFSD
many years ago who kept the volumne of the music down low so that
he had plenty of headdroom.  Unfortunately, this meant that the
natural hiss of the FM station drowned out quiet passages in the
music.  It would have been better that he had put the level up
and accepted the compression of the loud parts of the music which
is less objectionable than habving to listen to hiss on top of
the start of Bolero and other such pieces of music.  Look at what
is important and ignore the small stuff.  Better to have a nice
parabola of a slightly different focal length than a mirror that
is lumpy from poor mounting like the equal area mounting which
rolls down the edge and puts a hole in the center of the mirror.
Bob May
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