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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:
> *****
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> 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:
>
>> *****
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>> *****
>>
>> 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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