Re: [ATM] Schmidt corrector tools

["vladimir sacek" <vla@toast.net>]

George Anderson wrote:

> The sag for the corrector is very small ...

It may look small numerically, but keep in mind that it
is much more glass to remove than when parabolizing
that same (f/2) sphere. The max corrector depth is at least 
2/(n-1) times greater than depth of glass needed to 
remove from the edge and the center of that same sphere
in order to have it parabolized, "n" being the glass refractive
index. That is for the neutral zone at 0.707 radius; 
for the neutral zone at 0.866 radius the 
curve is at least 4.5/(n-1) times deeper.

I'm still puzzled by the neutral zone choice at 0.866 
radius. It requires more than twice deeper curve, and uneven
center edge level, which makes it considerable harder to
make than a corrector with 0.707 radius neutral zone.
It also has inferior non-optimized wavelengths correction, 
having their circle of least confusion coincide
with the optimum focus location. It is not a geometric blur
that gets imaged - it is diffraction pattern. And 0.707
neutral zone, by having best foci of non-optimized 
wavelengths coinciding with the optimum focus, results
in half the wavefront error for non-optimized wavelengths
vs. 0.866 neutral zone corrector, and accordingly less energy 
out of the Airy disc. It is at least twice easier to make,
and offers better correction; yet, it is not the usual choice.

What is funny (or sad, depending on the way you look at it), 
is that even professional opticians and designers tend
to favorize raytracing formalism over actual correction 
levels with Schmidt corrector.

Vlad (sorry for the direct response)


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