http://confocal-microscopy-list.275.s1.nabble.com/Astigmatism-aberration-as-a-function-of-distance-tp7582827p7582831.html
I agree, it could be the mirrors -are they pinched in some way or else you have a lens/source off axis.
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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
Mark B. Cannell Ph.D. FRSNZ