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To design a chromatically corrected optical system you need to use multiple
optics made of different types of glass.  The gist of it is that each glass
has slightly different wavelength-dependent indecies of refraction.  When
you put them together in the right way you can partly cancel out the
chromatically sensitive effects of the system.  The problem is that you can
usually only nail down two or three specific discrete wavelengths at a time.
 The simplest lens for this is an achromatic doublet.  It uses two types of
glass and has the same focal point for two specific wavelengths.  Operating
away from these two wavelengths will cause the focal point to drift.  The
better the design the less drift occurs, but it is really only 'perfect' for
two very specific wavelengths.  A triplet uses three pieces of glass
(typically) and nails down three specific wavelengths.  Basically you are
limited to designing for specific wavelengths.  The difficulty becomes which
specific wavelengths do you want to design for?  You can get 'good'
correction over a range, but you can only be dead-on for discrete values.
 You can get 'close' over a fairly wide range, but it takes a lot of design
work to pull this off.  This is further complicated by the fact that most
glasses have a steep change in optical properties towards the blue/violet/UV
portion of the spectrum, so if you are working in that range it becomes
trickier.  The balance between the different glass types and their
interactions with the overall design are highly complex, so as you make the
lens better it becomes astronomically more complicated and thus expensive.

Craig


On Mon, Jan 10, 2011 at 4:18 PM, Martin Wessendorf <[log in to unmask]> wrote:

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>
> On 1/10/2011 4:32 PM, Rosemary White wrote:
>
>  I've found that the red and blue emission aren't quite lined up
>> vertically,
>> at least with our "blue" 63x objective, so have to cut the top one or two
>> slices off a blue vertical series (depending on depth of slice), and the
>> bottom slices off a red series and reassemble to get the emissions aligned
>> -
>> i.e. from the same depth in the tissue.  The objectives do vary a bit,
>> perhaps ours isn't as well corrected as some.
>>
>
> Can anyone explain where the difficulty arises in correcting for axial
> chromatic aberration?  I've consistently seen serious axial chromatic
> aberration in good quality oil objectives from one very reputable
> manufacturer, between red, green and far-red (1 um off in the z-axis between
> green and red, and between red and far-red; 2 um between green and far-red).
>  I have always heard that axial chromatic aberration was easy to correct for
> and would've thought that a solution for 3-color correction would've been
> found 100 years ago.  However, I don't know enough optics to understand the
> subtleties.  Are there trade-offs to trying to obtain good correction,
> besides the cost in scattering of adding additional lens elements?
>
> Thanks--
>
> Martin
> --
> Martin Wessendorf, Ph.D.                   office: (612) 626-0145
> Assoc Prof, Dept Neuroscience                 lab: (612) 624-2991
> University of Minnesota             Preferred FAX: (612) 624-8118
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>