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[ATM] 3 point cell for 12 inch ?
Hello,
I am rebuilding the mirrorbox for my 12,5" Pyrex f4.8 with thickness
2"(51mm).
I got the advise that I should make 3 point cell.
But this type of cell I do not consider a Flotationcell, because there is no
rotational movement involved (like in a 6-point). Is
And is a 3 point with 0,44r still a good choice?? (I am not experienced, but
0,44r seems a bit small to me...)
I would like this to be confirmed before I start cutting aluminum and steel.
Harro Treur
Netherlands
-----Oorspronkelijk bericht-----
Van: Mark Holm [mailto:mdholm@telerama.com]
Verzonden: dinsdag 21 december 2004 4:31
Aan: atm@atmlist.net
Onderwerp: Re: [ATM] PLOP, new developments in mirror support, starte of,
the art edge support, etc
>
>
>Even if they did make my
>1/240th wave cell a 1/120th wave cell would I or
>anyone else be able to tell the difference?
>
One of the things ATM's have come to realize, at least some of us, in
the past 5 - 10 years is that there are ways to estimate, fairly
reliably, how much mirror (or other optic) error can be tolerated before
one might notice the difference. The beauty is that it really doesn't
require terribly complicated math. The RMS wavefront error can be fed
through fairly simple equations to get an estimate of the Strehl ratio
and the Strehl ratio can be, and has been, related to observational
experience.
Everybody who has gone to the trouble of testing it out on a well
characterized mirror says you can tell the difference between a Strehl
of 0.8 and 0.9+ From 0.9 to 0.95+, is probably getting near the limit
of detectability and above 0.95, probably nobody can reliably tell the
difference.
This plus Jeff Anderson Lee's results on likely effects of construction
tolerances on cell performance are what lead me to recommend a maximum
of 2.5 nm for RMS deformation in Plop. 2.5 nm all by itself is way
below detectability, but Jeff's work says that factors of 2 to 4
worsening are possible. 10 nm RMS gives a Strehl of not quite 0.94, so
I consider it at the edge of detectability. The 2.5 nm recommendation
should keep cell induced deformation below detectability in most cases,
even with reasonable construction tolerances thrown in. (Of course with
the caveat that Plop does not model lateral forces on the mirror.)
My RMS to Strehl calculator, using equations I cribbed from Jim Burrows,
is at http://pong.telerama.com/~mdholm/atm/cells/rmscalc.html
--
Mark Holm
mdholm@telerama.com
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