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I have to agree completely with what Beat says.

I'd also like to correct a mistake I made in a
previous posting - the spinning disk is of course
above the tube lens since it must be at the image plane.
It is, however below the eyepiece so if the eyepiece
is involved in the correction (as it was on most
160mm tube length systems) there will be a problem.

Here at the EMBO course some manufacturers were
showing actual curves of focus divergence with
wavelength (which was nice) and it's clear that
up to 500nm within the visible range is quite a
possible figure.  If this seems bad reflect that
it's only around 0.025% while the refractive index
is changing by 1%.  One point that was very obvious
was that once out of the corrected range (eg in the
violet) you are going to be very much adrift.  So
use a violet corrected lens if you're working at 405
or 440nm.

                                         Guy


> 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
>


--
Associate Professor Guy Cox
Electron Microscope Unit,
University of Sydney,
NSW 2006, Australia

Phone:+61 2 9351 3176    Fax:+61 2 9351 7682
http://www.guycox.net