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Re: [ATM] Crazy simple mirror cell idea
Robert,
> This demonstrates very nicely the "square of diameter" law (all other
> things being proportional):
>
> 10 nm * (180/670)^2 = 0.7 nm
>
> No need to run the second Plop analysis :D.
Not if you have done the first run. If you have a new project, though, it is
a good idea to play around with PLOP to see what you are up to.
Talking of the subject - the effect on light concentration in the
diffraction peak - also known as the Strehl ratio - is affected by the mean
square deformation, not RMS-the root of it.
BTW, most errors are "independent" in the sense that they add to a RMS
total - say, you have 7 nm RMS of spherical aberration, 5.5 nm RMS of
astigmatism and 4.5 nm RMS of cell deformation: the squares add up to ~100,
meaning the total RMS error is 10 nm RMS (rather than 17 nm) - and half of
the total error comes from the sph.abb.
A simple memorizable approximation: a 10 nm RMS surface deformation (or 100
square nanometers MS if you like ;-)corresponds to twice that on the
wavefront, and assuming 550 nm green light, this deformation would lower the
Strehl ratio by 0.2 (from 1.0 to 0.8 for a perfect mirror, or from 0.9 to
0.7 for a less perfect one). 5 nm will lower by (5/10)^2*0.2 or by 0.05
which is not readily noticeable - 2.5 nm by 0.0125.
It may make sense to aim for this when designing a cell for a quality
mirror - if nothing else, it will give you some safety margin. A PLOP test
run of the original 14" mirror, under reasonable assumptions, would give
IIRC about 6.6 nm RMS error, and lower the Strehl ratio by 0.08 or so. Not
obvious, I believe, but I'd personally play it safe and use a 6 pt cell in
his place.
And perhaps even better: now we have the "orthogonal counterpart" to PLOP:
http://www.cruxis.com/scope/mirroredgecalculator.htm
to calculate the deformations of edge supports.
While , obviously, both are approximations to reality, they will no doubt
give very reasonable indications of what to expect and what to select.
Perhaps surprisingly to many (it was indeed to me), a 2 bearing support,
correctly placed, will give a totally negligible deformation to any but the
largest amateur mirrors....
Nils Olof
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