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Hi Mike,

GDD is the group delay dispersion, the difference in time between different
wavelengths of light when passing through a system.  It represents the
tendency of shorter wavelength components of a pulse to travel at a
different speed through glass than longer wavelengths.  Its is the temporal
analog of chromatic aberration if you are familiar with that concept.

The GDD depends on:

1)  The amount and type of glass in the system (and therefore on the
objectives used)
2)  The wavelength of light (typical glasses have lower GDD as one
approaches 1.3 um).
3)  The length of the pulse of your laser (short pulses are very sensitive
to GDD)

In your case, your chameleon has a relatively long pulse length (140 fs),
and will therefore not be very sensitive to even relatively large amounts
of GDD.  Personally, I do not bother compensating GDD on our Chameleon
because I found that it had a very minor effect on image contrast (although
I am using scan optics that are optimized for low GDD).

However, since you have GDD correction built in, you might as well use it.
Take a slide filled with the fluorophore of your choice and use your
preferred objective + wavelength.  Then adjust GDD until the image
brightness is maximized.  I recommend using low power and concentrated
fluorophore to minimize photobleaching.  You can repeat this for each
objective and wavelength if you are determined, but the difference will
likely be insignificant unless you are using very widely spaced wavelengths
or extremely dispersive objectives.

Hope that helps,
Mike


On Wed, Nov 12, 2014 at 3:44 PM, Michael Moore <[log in to unmask]> wrote:

> *****
> To join, leave or search the confocal microscopy listserv, go to:
> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> Post images on http://www.imgur.com and include the link in your posting.
> *****
>
> I'm a new member to the confocal listserv
>
> Can someone explain GDD in regards to multiphoton imaging?
> And how is GDD adjusted.
>
> We currently have a Coherent chameleon vision II that has GDD.
>
> I have been told conflicting instructions on how the GDD is adjusted or
> used for imaging.
>
> Maybe I just don't understand how GDD works.
>
> for one, is GDD adjusted for each objective used?  is it independent of the
> objective?
>
> Thanks to all who reply.
>
> Mike
>