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Re: [ATM] How much cell induced deformation is too much?
Nils Olof Carlin wrote:
>So, if we would let the cell lower by say 1% (to 0.99 for a perfect mirror,
>or from 0.95 to 0.94 for a more realistic one), how much RMS wavefront
>error - assuming a reference wavelength of 550 nm, near the peak of photopic
>vision? I get some 8.7 nm, or a little more than 4 nm on the surface. This,
>4 nm RMS, I think could be a practical upper design limit for PLOP, allowing
>some realistic margin for less than perfect implementation. But then again,
>this is quite arbitrary.
>
Plop is frequently run with 4.25e-6 mm (4.25 nm) RMS as an acceptable
threshold. This however does not take into account construction errors.
In my study on 18 point cells I found that construction errors on the
order on 1 mm to 2 mm could seemingly result in varying amounts of
performance changes, depending on the cell design. For non-refocus
optimized cells, the Monte Carlo error simulation resulted in about a 2x
degradation on average performance and a 4x-5x performance degradation
in the worst case. For the refocused cell design, the average
degradation was over 3x and the worst case was over 8x.
1000 run Monte Carlo on refined models with refocusing
DesignRMS AvgRMS MaxRMS Avg/Des Max/Des
Straw-man 2.67E-06 4.84E-06 1.05E-05 1.81 3.95
Even force 2.65E-06 4.82E-06 1.05E-05 1.82 3.96
No refocus 2.36E-06 4.87E-06 1.09E-05 2.06 4.62
0.8 force_ 2.37E-06 5.29E-06 1.20E-05 2.23 5.07
Refocused 1.59E-06 5.45E-06 1.34E-05 3.43 8.43
Thus construction imprecision can significantly affect the cell
performance. The study also concluded that part balance errors are more
critical than part placement errors, so it is very important to
construct the parts carefully.
Hence I would probably recommend a limit of 2 nm (2.0e-6 mm) for
non-refocused cell designs and 1.0 nm to 1.5 nm (1.03-6 to 1.5e-6 mm)
for refocused cell designs, unless precise machining is used to generate
the parts.
Jeff Anderson-Lee
http://elib.cs.berkeley.edu/~jonah/18plus/p18.html
A Study of 18-point Mirror Cell Optimization Using Varying Forces
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