[ATM] Re: Weird Foucault readings

[Vladimir Sacek <vladis.2@juno.com>]

Yes, I did run into that article some time ago. Nils tried to determine
at what point of zonal defocus the zonal null is likely to be most
"convincing" visually. Both, "non-linearity" (uneven illumination across
the zone) and reversed illumination change for the two opposite zones
result from the zonal defocus, which causes different areas of the zone
to be affected differently (illumination-wise) for any given position of
the KE. All three suggestions for the correct zonal radius are around the
best focus (also called diffraction focus) location. Couder is right on
with (h^2+z^2)/2R, while Texereau and Carlin are slightly off. But the
difference is rather negligible: for a 16" f/4.5 it would come to less
than 1/40 wave wavefront error at ~70% zone. Much more substantial error
is already built in by averaging out the entire zone to its mean radius.

What the article doesn't address is what is the error margin in dermining
the most convincing null that is imposed by the objective perception
limitations of an average individual (with necessary skills assumed) in
real-life conditions. Part of those real-life conditions is less than
perfect mirror, with less than symmetrical zonal surfaces which changes
defocus structure and affects zonal illumination in an unpredictable
manner. 
Consequently, while we could try to approximate error margin for a
perfect or near perfect mirror, it seems to be pretty much unpredictable
for other - and rather common - scenarios. 

Vlad
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