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I like to make some (hopefully correct) comments and also raise a number of
(possibly naive) questions:

As far as I have seen, it has not been mentioned that the dispersive index
of the immersion medium critically affects CA. So if the DI of the
immersion fluid doesn't agree with what the objective has been designed
for, you can get dramatic CA even if the RI, which is defined for a
specific wavelength, is correct. Which brings me to a number of questions:
Why is the DI not usually specified for immersion fluids? Cargille mentions
on their web site that the DIN/ISO standard DI is 44 for immersion oils,
but from data that I have seen recently, at least some objectives and/or
immersion oils do not seem to adhere to that standard. And what about the
temperature coefficient of the DI? Is this something to consider also? And,
to make things even more complex, what about the DI (and its temperature
coefficient) of the specimen itself? Is anybody taking potential DI
mismatch into consideration? Or can it simply be neglected?

I also have another question to the optics pros here on the list: Most
people seem to assume that longitudinal CA is planar, i.e. constant across
the field. Is this really true (I have a gut feeling that it is not)? If
not, post-acquisition correction of longitudinal CA might be rather
complex. It would also limit the usefulness of having channel-specific
adjustable pinholes, wouldn't it?

Cheers,

Beat