[ATM] Re: Maksutov vs. Schmidt

[vladimir sacek <vladis.2@juno.com>]

>However, this is the second time I've seen a symptom.  I know the
theory suggests that putting the neutral zone at root(3/4) "minimizes"
color, as the slope, equal at the 0.5 and 1.0 zones is equal (but
opposite
in sign)... however, it stinks!(IMO)  If the neutral zone is  root(1/2),
the RMS wavefront error appears halved. <

Richard,

At first I thought you're talking nonsense, but after taking a little
closer look, 
I'd say you made a great observation! What the usual calculation goes for
is determining Schmidt shape which will produce smallest chromatic blur
at the green focus. So what the 0.866 neutral zone does is placing the
circle of
least confusion of other colors than green (which is corrected for s.a.
by default)
at the green (optimum) focus. But the smallest blur location is not the
best focus location
in presence of spherical aberration. At the midway between paraxial and
marginal foci,
the RMS error is only half as large, despite twice larger blur. And that
is exactly what
the 0.707 zone does: it places best aberrated colored focus at the
optimimum (green) 
focus. It definitely gives best color correction visually, having the
least amount of energy
lost to the Airy disc. 

I checked it out on the 500mm f/16 Schmidt-Cassegrain, and it confirms
this reasoning. 
The amount of spherochromatism remains practically unchanged for all
three positions 
of the neutral zone (0, 0.707 and 0.866), it is only position of the
green focus within aberrated 
color foci that changes. For the 0 neutral zone height, optimum green
focus coincides with 
paraxial color foci, just as it is obviously determined by the neutral
zone position.

Blur size itself may be a factor for some photo/CCD applications but, in
general, 0.707 Schmidt
should allow highest resolution having roughly 80% of the energy
concentrated in the blur
core which is about 2/3 of the size of the circle of least confusion.

Schmidt corrector with the neutral zone at 0.707 radius is easier to
make, not only due to more than 
twice smaller depth, but also due to the edge and the center having the
same height. Isn't that true,
that our beginnings never know our ends? You are advocating Maksutovs,
but your observation
significantly helps Schmidts' cause :)

Vlad
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