"Michael A. Covington" wrote in message ...
Is that also true in Mak/Cassegrains?

It should be, because -- like an SCT -- the Maksutov is using a single layer
of glass to correct spherical aberration.

No, it should not. It is not difficult to understand why a menisk can be made
to suffer far less from chromatic aberrations (spherochromatism), while Schmidt
plate can not. ("can" is a critical term - not all Maksutovs designs will
be free from spherochromatism)
It does require a bit deeper understanding of optics, maybe a step up from
"single layer of glass" analogy.

Why doesn't the corrector plate "correct" for this in an SCT.

It is only one layer of glass. In the process of correcting spherical
aberration (which is its purpose), it introduces slight chromatic aberration
(because all glass does this, unless you use multiple layers of glass to
cancel each other out). Glass bends different wavelengths of light
differently.

The last sentence is the only correct one. "Glass bends different wavelengths
differently".
Let's have a mental experiment.
Observe a ray of white light hitting a menisk in a Mak scope. It bends
outwards (away from optical axis) and spreads into components. Violet and
blue components will bend the most. Red and IR will bend the least. Now
let's see what happens when this fan hits the rear surface of the menisk.
Violet/blue entrance point is further away from the axis than red/IR one.
All rays will bend INWARDS (towards the axis). But now let's remember the rule -
violet/blue rays will bend MORE. As a result the slope of blue rays is steeper
than that of the red ones. They will CROSS somewhere. Can we make this in
such a way to make this crossing coincide with focus point ? Of course we can.
Result ? Our "single layer of glass" does NOT introduce any chromatic
aberration, even if it has some power. We have invented an achromatic menisk.
If only someone else didn't do the same more than 60 years ago. His name
is Dmitry Maksutov.

Can we apply the same logic to Schmidt plate ? No. Why ? Because Schmidt plate
is a compound curve that is positive lens in the middle and negative at the
edges at the same time. Hence spherochomatism cannot be avoided. It can be
minimized by having a neutral zone placed at 87%, but that's it. It will
never be zero (strictly speaking it will never be zero in a Maksutov, but
say properly designed Maksutov Newtonian can have chromatic residuals,
including spherochromatism, that even today's best triplet APOs can only
dream about).

Frankly, in my experience the chromatic aberration of an SCT is very, very
small, much less than that of a refractor, and I have never been able to
observe it visually (even when doing sensitive star-tests). I have also not
heard of visible chromatic aberration in a Mak/Cass.

Spherochroomatism looks vastly different from primary or secondary chromatic
aberrations. You just have to learn to detect it, and then you're cursed
forever :-)

Bratislav