Re: Astigmatism aberration as a function of distance

Posted by Lu Yan on
URL: http://confocal-microscopy-list.275.s1.nabble.com/Astigmatism-aberration-as-a-function-of-distance-tp7582827p7582833.html

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Hi Kyle, Mark,

Thanks for your suggestions. Regarding to Kyle's comments/questions:

1) my beam splitter is those cube beam splitter. I should have mentioned
that even without beam splitter, when I imaged the collimated beam through
a lens far away from the collimation lens [between two lenses are just two
mirrors (1 inch Thorlabs silver protected mirror)], I got similar amount of
astigmatism. [To Mark: would mirrors usually also cause astigmatism in
microscopy? Then does that make sense to use larger mirrors? my beam 1/e^2
size is about 5 mm in diameter.]

2) the fiber is single moded, and I have tried imaging is using a 150X
Nikon objective lens (single lens imaging), and it does not have higher
order mode, pretty Gaussian. BTW, imaging with 150X obj. did not give me
astigmatism as I moved the fiber (or the camera) back and forth.

Besides these, I have another question about astigmatism. Supposed that I
have an off-axis lens for the collimation of my Gaussian illumination beam,
does the amount of astigmatism I will get depend on the distance after
which I will image the Gaussian beam?

Thanks,
Lu

-----------------------------------------------------
Lu Yan
Nanostructured Fibers and Nonlinear Optics Laboratory
Electrical and Computer Engineering
Boston University
8 St. Mary St., Boston, MA, 02215
[phone]
[hidden email]
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On Wed, Nov 5, 2014 at 3:04 AM, Mark Cannell <[hidden email]>
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 agree, it could be the mirrors -are they pinched in some way or else you
> have a lens/source off axis.
> HTH
>
> Mark
>
> On 5/11/2014, at 7:56 am, Kyle Douglass <[hidden email]> 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.
> > *****
> >
> > 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
> Professor of Cardiac Cell Biology
> School of Physiology &  Pharmacology
> Medical Sciences Building
> University of Bristol
> Bristol
> BS8 1TD UK
>
> [hidden email]
>