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Is the reflective side of the dichroic facing the incoming light? If not,
you might get astigmatism in reflection even from a perfectly flat dichroic
in a non-infinity space.
On Tue, Feb 14, 2017 at 10:51 AM, Alfred Millett-Sikking <
[log in to unmask]> wrote:
> *****
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> *****
>
> Hi Kyle,
>
> Our friends and colleagues have done an excellent job of covering this
> topic, and I very much agree with the contents and summaries provided. I
> would like to add one further minor point which may or may not be
> applicable depending on your setup:
>
> WFE of any type (including astigmatism) is additive and you mentioned
> upstream dichroics and filters (and mirrors?). If you're unlucky then it
> may be that these previous optics have already put you 'on the edge' of an
> acceptable/perceptible astigmatism and that the last guy is simply pushing
> you over. If you can't fix it with careful mounting of the last guy then it
> may be worth giving the upstream optics an equal measure of care
> (especially if you have reflectors). Without a TWE measurement of the
> system optic by optic you cannot tell what's going on, so just be
> suspicious of everyone!
>
> Regards,
>
> Alfred Millett-Sikking.
>
>
> On 14 February 2017 at 06:35, James Kerin <[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.
> > *****
> >
> > COMMERCIAL RESPONSE
> >
> > Our President and Managing Director have prepared the following response:
> >
> > Hi Kyle
> >
> > Based on our experience of designing image splitters and multicamera
> > adapters, we can strongly underline the points made by Jeff Carmichael.
> As
> > he says, to avoid any aberrations in the reflected channel, the dichroic
> > must be “softly” mounted against a truly flat surface. Curvature in only
> > one direction will indeed give astigmatism, but more complex curvatures,
> > especially if there is a bit of twist as well, can give significant image
> > distortion too – think fairground mirrors! Even if the images are more
> or
> > less in focus over the field, this will clearly show up when (as is so
> > often required) the transmitted and reflected images are superimposed for
> > analysis. As camera resolution has improved, such effects have become
> ever
> > more obvious, but even with megapixel cameras it should be possible to
> > overlay the images to within a just a pixel or two over the field.
> However
> > that does require similar detail to the design of the rest of the optical
> > system, which should be as symmetrical as possible between the
> transmitted
> > and reflected pathways, as it is in our image splitters as well as our
> > camera adapters.
> >
> > The flatness issue applies whether the reflection occurs in an infinity
> > space or an imaging space, but there is another problem if the dichroic
> is
> > in imaging space, namely that of astigmatism in the transmitted channel.
> > This is because the dichroic is at an angle (typically 45 degrees of
> > course) with respect to the light path. Optical modelling shows the
> > following effects, some of which may be counterintuitive. As might be
> > expected though, the astigmatism, which is uniform over the field,
> > increases with the refractive index and thickness of the dichroic, as
> well
> > as with its angle. (By the way, that's probably why the dichroics used
> in
> > microscopes to introduce an epi-illumination beam are traditionally
> “thin”,
> > typically just a millimetre, as in the older (noninfinity) microscopes
> the
> > dichroic was in the imaging space from the objective).
> >
> > We wonder if this effect might be relevant to Kyle's problem, especially
> > if the dichroic is relatively thick. Relating to that, we have an
> > observation and a suggestion. The optical modelling shows that the
> > astigmatism is independent of the dichroic's position within the imaging
> > space, so moving it closer to either the camera or the focussing lens
> won't
> > help.
> >
> > We are also pleased from our modelling to confirm that Andrew York's
> > solution to use a second piece of glass mounted at an orthogonal angle
> is a
> > correct one rather than just a “hack”, even though it may “sound” wrong.
> > The astigmatism arises because the effective thickness of the glass is
> > different in the x and y directions because of it being at an angle. The
> > sign of the angle doesn't matter, so adding another piece of glass angled
> > oppositely would make the problem worse rather than better. But it you
> > rotate by 90 degrees, the directional differences now cancel, and you're
> > just left with a small and probably negligible (according to our
> modelling
> > at least!) chromatic effect from the dispersion of the glass.
> >
> > Dr MV Thomas & Jez Graham
> > Cairn Research Ltd
> > Graveney Road
> > Faversham
> > Kent, ME13 8UP
> > UK
> > Cairn Research is a European scientific instruments manufacturer based in
> > Kent, UK.
> > www.cairn-research.co.uk
> >
> >
> >
> >
> >
> > On 2/13/2017 9:34 PM, Andrew York 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 worried that your transmission path will also have astigmatism.
> >> Even a
> >> perfectly flat piece of glass yields astigmatism in transmission if it's
> >> tilted in a non-infinity space, right?
> >>
> >> (Warning: lousy advice below)
> >> I've encountered this myself, and "fixed" it with a second piece of
> >> glass,
> >> the same thickness and material as the first, tilted equally about an
> >> orthogonal axis, to introduce "equal and opposite" astigmatism. This
> was a
> >> hack, but it was good enough for my needs at the time. Depending on your
> >> needs, perhaps you could use tilted flat glass as a tunable astigmatism
> >> compensation.
> >>
> >> On Mon, Feb 13, 2017 at 8:45 AM, Rusty Nicovich <
> [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.
> >>> *****
> >>>
> >>> Kyle,
> >>>
> >>> The other poster was me. I ultimately fixed the issue to my
> satisfaction
> >>> by carefully mounting the dichroic to the cube with double-sided tape
> >>> rather than the provided clamp. That method was stress-free enough to
> >>> not
> >>> induce any undue curvature in the dichroic, at least qualitatively as
> no
> >>> astigmatism immediately obvious in the reflected image. This was even
> >>> with
> >>> a relatively thin (1.1 mm) substrate for the mirror. This solution is
> >>> similar to what Jeff suggested using silicone adhesive instead of tape.
> >>>
> >>> Another fun part of that system is that the camera fans would induce
> >>> quite
> >>> a large vibration and only along one axis. The vibration period was
> >>> 10-20
> >>> ms, which meant any longer exposures would yield a PSF elongated on
> that
> >>> axis. Astigmatism would vary along the focal axis and this didn't, but
> >>> it
> >>> still gave an asymmetric PSF at the focal plane.
> >>>
> >>> Thanks,
> >>> Rusty
> >>>
> >>> On Mon, Feb 13, 2017 at 7:49 AM, Jeffrey Carmichael <
> >>> [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.
> >>>
> >>>> *****
> >>>>
> >>>> COMMERCIAL RESPONSE
> >>>>
> >>>> Hi Kyle,
> >>>>
> >>>> We've found that it's most often the mounting of the dichroic itself
> >>>> that
> >>>> causes the most distortion in an image reflected off of a dichroic.
> You
> >>>> could theoretically have an "ideal" dichroic with no curvature, but
> once
> >>>>
> >>> it
> >>>
> >>>> is affixed, it will be torqued out of flatness to some extent unless
> you
> >>>> can completely avoid any mounting stress.
> >>>>
> >>>> If held by means that apply pressure on top or bottom of the large
> >>>>
> >>> surface,
> >>>
> >>>> you will get various forms of non-spherical astigmatism. Even light
> >>>> pressure on the sides of the dichroic will have some effect.
> >>>>
> >>>> As Zdenek suggested, thicker dichroics are stiffer and will better
> >>>> resist
> >>>> deformation.
> >>>>
> >>>> If possible, try using something like silicone RTV applied very
> >>>> sparingly
> >>>> in small beads around the outside edges of the dichroic, but never
> under
> >>>> it. This serves to prevent the dichroic from moving around without
> >>>> applying stress. If the "holder" is removable, then you can swap out
> >>>>
> >>> these
> >>>
> >>>> mounted dichroics and not fiddle with the dichroic itself once
> affixed.
> >>>>
> >>>> Jeff
> >>>>
> >>>>
> >>>> *Jeff Carmichael*
> >>>>
> >>>> *Technical and Product Marketing Manager*
> >>>>
> >>>> *[log in to unmask] <[log in to unmask]>*Chroma Technology
> >>>> Corp.
> >>>>
> >>>> *an employee owned company*
> >>>> *10 Imtec Lane*
> >>>> *Bellows Falls, VT 05301*
> >>>> *802-428-2528 Office*
> >>>> *802-428-2528 Fax**800-824-7662 Toll Free*
> >>>>
> >>>> On Mon, Feb 13, 2017 at 9:27 AM, Kyle Douglass <[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.
> >>>>
> >>>>> *****
> >>>>>
> >>>>> Hi everyone,
> >>>>>
> >>>>> A couple years ago there was a post on this forum about significant
> >>>>> astigmatism appearing in the reflected path of a custom-built, two
> >>>>>
> >>>> camera
> >>>
> >>>> TIRF setup. (http://confocal-microscopy-list.588098.n2.nabble.com/
> >>>>>
> >>>> Reflec
> >>>
> >>>> ted-image-gt-astigmatism-td7584402.html) The original poster had
> >>>>>
> >>>> placed
> >>>
> >>>> a
> >>>>
> >>>>> dichroic before a pair of matched tube lenses to split the two color
> >>>>> channels onto two separate cameras. The general consensus was that
> you
> >>>>> could "buy your way out of the problem" by buying a thick, ultraflat
> >>>>> dichroic.
> >>>>>
> >>>>> I am trying a similar approach on a setup in our lab but, due to
> space
> >>>>> constraints that are not easily overcome, have tried first placing
> the
> >>>>> channel-splitting dichroic in the image space after the final tube
> lens
> >>>>> instead of in the infinity space between the objective and tube lens.
> >>>>>
> >>>> Even
> >>>>
> >>>>> with a 3 mm thick ultraflat dichroic, I see moderately bad
> astigmatism
> >>>>>
> >>>> in
> >>>
> >>>> the reflected channel. I have ruled out other possible sources of the
> >>>>> astigmatism, like an additional dichroic for the excitation light and
> >>>>> filters that already lie in the infinity space between the objective
> >>>>>
> >>>> and
> >>>
> >>>> tube lens.
> >>>>>
> >>>>> My question is: is it even worth trying to eliminate the astigmatism
> in
> >>>>> the reflected path of a split-channel setup if the dichroic does not
> >>>>>
> >>>> lie
> >>>
> >>>> in
> >>>>
> >>>>> the infinity space? Or is it pretty much always a bad idea to put the
> >>>>> splitter dichroic in the image space?
> >>>>>
> >>>>> My current suspicion is that the tolerances in the alignment have to
> be
> >>>>> very, very tight to avoid astigmatism with the splitter in the image
> >>>>>
> >>>> space.
> >>>>
> >>>>> Thanks for your feedback.
> >>>>>
> >>>>> Kyle
> >>>>>
> >>>>> --
> >>>>> Kyle M. Douglass, PhD
> >>>>> Post-doctoral researcher
> >>>>> The Laboratory of Experimental Biophysics
> >>>>> EPFL, Lausanne, Switzerland
> >>>>> http://kmdouglass.github.io
> >>>>> http://leb.epfl.ch
> >>>>>
> >>>>>
> > --
> >
> > *James Kerin
> > *
> >
> > *Marketing Director*
> > Cairn Research Ltd
> > Graveney Road
> > Faversham
> > Kent, ME13 8UP
> > UK
> >
> > Direct: + 44 (0)1795 594507
> > Fax: + 44 (0) 1795 594510
> >
>
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