Posted by
Kyle Michael Douglass on
URL: http://confocal-microscopy-list.275.s1.nabble.com/Astigmatism-aberration-as-a-function-of-distance-tp7582827p7582830.html
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Hi Lu,
On 11/05/2014 05:36 AM, Yan, Lu wrote:
> Hi folks,
>
> I am building a fiber based confocal microscopy setup (with sample stage
> scanning). But I always got some astigmatism aberration in PSF measuremnts.
> The similar aberration was there even I replaced the objective lens with a
> regular lens and imaged my illumination beam through that lens with a
> camera. I got elongated beam 'spot' on both sides of the focal plane, and
> the orientation of the two 'spot' were orthogonal. I think that is
> astigmatism aberration if I am not mistaken. I draw a schematic in Evernote
> so I can include it here. Here is the link:
>
https://www.evernote.com/shard/s275/sh/55130807-98d4-4748-a4a9-64d19650b695/be0756284a13da18fe6d1f7f419cbcfe> (copy and paste if the link does not work in email)
>
> I tried to adjust both lens in xy to avoid off-axis incident, but the
> aberration would go away. So I got confused where they came from. I hope
> someone here could lead me a direction to further look into it.
>
> Thanks very much,
> Lu
Here are two quick checks that came to mind. Maybe you've tried them,
but it's still good to check all the possible sources of the problem:
1) Is the beam splitter really a glass cube as it's drawn, or one of the
thin dichroic-like beam splitters? Sometimes the thin dichroics are
slightly warped in one direction and can introduce astigmatism after
reflection.
2) When you illuminate a flat surface or white piece of paper with light
directly coming from the single mode fiber, do you see a smooth,
homogeneous intensity distribution or some other kind of structure? I've
noticed that one can occasionally get a weak first-order mode coming
from the fiber if the wavelength is close to the multimode cut-on. It
may not necessarily lead to astigmatism, but it's still good to check.
Good luck!
Dr. Kyle Douglass
The Laboratory of Experimental Biophysics
EPFL, Lausanne, Switzerland