Strange flare in Nikon CF N plan apo 20/0.75 objective

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microcosmos
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Strange flare in Nikon CF N plan apo 20/0.75 objective

#1 Post by microcosmos » Sun Jul 11, 2021 4:34 am

I am having a problem with my Nikon CF N plan apo 20/0.75 160/0.17 objective, which I recently acquired second-hand in mint condition.

It produces a nice and clear transmitted bright-field image in the eyepiece with a thin section of onion root.

However, when I view a piece of paper (about 100 μm thick) using the same transmitted bright-field, there is a lot of flare when the surface of the paper is in focus. It is quite bad and looks like fogging.

When I switch to my semi-plan achromatic 10/0.25 objective next to it on the turret, the image is very crisp and has no flare at all.

Both onion root and paper have a coverslip on top.

I am using Köhler illumination, with both the field diaphragm and condenser iris diaphragm stopped down appropriately.

The lens elements in the plan apo objective look perfectly clear, with no fogging or stains, just a few tiny dust particles.

The microscope I am using is the Radical RPL-3T. It has an incident illuminator so there's a half-mirror in the optical path. It also has a polarizer, analyzer and Bertrand lens, but these were out of the optical path.

I am aware that light scattered by out-of-focus parts of a thick sample can cause flare. However, I have viewed similar paper samples under a Nikon Eclipse Ni-E with a 20× plan apo lambda objective, and the image was perfectly good.

Does anyone have any idea what could be causing the flare? Could it be something in the microscope that is incompatible with a high-NA objective but only becomes visible with thick samples with uneven surfaces?

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#2 Post by PeteM » Sun Jul 11, 2021 4:42 am

One possibility is flare, as you suggest, introduced by the thicker section. Apos have more lens elements and thus are more susceptible to internal reflections. You might try carefully stopping down the field and condenser irises for better control - but that thick section may be introducing it's own scattered light?

The more recent Apos will have better coatings. It wouldn't be a surprise if it controlled internal reflections better.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#3 Post by microcosmos » Sun Jul 11, 2021 5:27 am

You are probably right, although it looks worse than I expected after accounting for all the factors.

I tried stopping down both diaphragms all the way, but it didn't have any appreciable effect on the flare.

I will try to find a chance to test the objective on another microscope and vice versa to see what happens.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#4 Post by MichaelG. » Sun Jul 11, 2021 5:50 am

This is a long-shot [especially as your sample is on a cover-slipped slide] but may I suggest you try some different paper samples

Many years ago, when working at KODAK, I was introduced to the difficulties of photographing materials that contained ‘optical whiteners’ … it is just possible that your paper is glowing.

Try looking at an un-bleached, non-whitened, paper sample
[ unless, of course, that is what you already have ]

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Ref. https://en.wikipedia.org/wiki/Optical_brightener
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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#5 Post by microcosmos » Sun Jul 11, 2021 7:11 am

Yes I was looking at a piece of paper (a postage stamp) that had a coating containing calcium carbonate and some other optical brightener that fluoresces in long-wave UV.

Alas, that doesn't seem to be the problem. Following your suggestion, I took a look at a piece of uncoated and unbleached buff-coloured paper but the flare was just as bad.

Many of the paper samples I was looking at under the Ni-E with the plan apo lambda objective also had the brightener, but they didn't flare.

Nevertheless, thanks for raising this. I would like to explore every possibility no matter how remote.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#6 Post by MichaelG. » Sun Jul 11, 2021 8:31 am

microcosmos wrote:
Sun Jul 11, 2021 7:11 am
Yes I was looking at a piece of paper (a postage stamp) that had a coating containing calcium carbonate and some other optical brightener that fluoresces in long-wave UV.

Alas, that doesn't seem to be the problem. Following your suggestion, I took a look at a piece of uncoated and unbleached buff-coloured paper but the flare was just as bad.

Many of the paper samples I was looking at under the Ni-E with the plan apo lambda objective also had the brightener, but they didn't flare.

Nevertheless, thanks for raising this. I would like to explore every possibility no matter how remote.
.
Thanks for the very thorough response
Still clutching at straws … I’ve just had a look at the Nikon brochure, and noted this:
The chromatic-aberration correction capabilities of these new states of the art objectives extends across the entire visible spectrum to include the g line(violet)
and I wonder what light source you are using in the incident illuminator
The microscope I am using is the Radical RPL-3T. It has an incident illuminator so there's a half-mirror in the optical path. It also has a polarizer, analyzer and Bertrand lens, but these were out of the optical path.
… or even if there is an ‘unfortunate’ reflection from a back surface in the objective.

MichaelG.
.

Edit: __ for the convenience of others:
https://www.radicalscientific.com/polar ... scope.html
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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#7 Post by microcosmos » Sun Jul 11, 2021 9:49 am

I was using transmitted light for the observation and the incident illuminator was turned off. But I was wondering if the half-mirror in the optical path might have something to do with the flaring.

Both the transmitted and incident light sources are 6 V 20 W tungsten halogen lamps.

The microscope has several mechanical and optical issues which I detail here, but I can't isolate any potential cause of the flaring. The strange thing is that it doesn't flare with the other objectives and it doesn't flare with thin sections.

I certainly wasn't expecting this objective to flare up, given its credentials! But all the state-of-the-art aberration corrections may not necessarily immunize it from the ‘unfortunate’ reflections that you mentioned, due to some unknown design quirk.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#8 Post by MichaelG. » Sun Jul 11, 2021 1:02 pm

Sorry, I’m out of ideas for now :(

I will look at your excellent web-page later … and maybe find inspiration.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#9 Post by microcosmos » Sun Jul 11, 2021 1:12 pm

No problem, thanks to you and Pete for offering possible explanations. I am still on this and will report back if I discover anything.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#10 Post by BramHuntingNematodes » Sun Jul 11, 2021 2:17 pm

I wonder about that half mirror as I have an epi darkfield microscope that produces a similar flare in bright field for certain materials
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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#11 Post by Scarodactyl » Sun Jul 11, 2021 5:18 pm

It might be worth taking the head off and looking down the illumination path directly to see if you sppt any hotapots that could be causing flare.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#12 Post by patta » Sun Jul 11, 2021 6:22 pm

maybe if you "mount" the paper soaked in water under the coverslip
so it gets flattened and reflections/scattering from fibers is reduced because water immersion

Ok not feasible for valuable stamps.

NA 0.75 is serious monster aperture, would expect that rough paper surface isn't flat enough and out of focus fibers hanginv out, do big flare

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#13 Post by microcosmos » Mon Jul 12, 2021 10:44 am

Thanks everyone for the suggestions. I'm working through them one by one.
patta wrote:
Sun Jul 11, 2021 6:22 pm
maybe if you "mount" the paper soaked in water under the coverslip
so it gets flattened and reflections/scattering from fibers is reduced because water immersion

Ok not feasible for valuable stamps.

NA 0.75 is serious monster aperture, would expect that rough paper surface isn't flat enough and out of focus fibers hanginv out, do big flare
I used a piece of scrap paper with and without water under the coverslip. No difference, it still flares with water.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#14 Post by microcosmos » Tue Jul 13, 2021 9:38 am

I removed the analyzer assembly (which comprises the analyzer, a compensation plate slot and a Bertrand lens slot) and the epi-illuminator, and connected the head directly to the turret. That finally got rid of the flare and I got an apo-quality image of the paper surface from the plan apo objective.

The flare returns if I reinstall either the analyzer assembly or epi-illuminator, but it seems worst when both analyzer assembly and epi-illuminator are in place.

When I look down the tube, some of the circular apertures in the analyzer assembly can be seen protruding slightly into the light and reflecting it.

As for the epi-illuminator, the half-mirror is too small and you can see the glue on the edges being lit up in the tube. The glue wasn't applied carefully and some of it flowed onto the edges of the glass. The glass has fine cleaning scratches across it so it is visible when you look down the tube.

Perhaps the reflective obstructions in the light path, the extended length of the light path (with analyzer and epi-illuminator), the large NA and complicated lens arrangement of the objective and the thickness of the paper sample all conspired to cause the flare.

Could the finite correction optics and smaller RMS thread of my objective somehow be contributing factors to the flare, perhaps through diffraction effects or interactions with the tube, or are they completely irrelevant? I had no flare with paper samples on the Ni-E with an infinity-corrected lambda plan apo objective with a larger-diameter thread.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#15 Post by patta » Wed Jul 14, 2021 9:25 am

So some more guessing, after your observations.
See the illustration below.
I have a Nikon 20x 0.75, CFI, assume is somehow similar to your CF N.
This objective has a huge back lens, larger than any other objective I've seen. This is because of its low mag / high NA.
So it outputs a light cone (blue) much larger than normal objectives (red); the light at higher incidence angle may then bypass the baffles and hit the sides of prisms or plates - while wouldn't do that with more normal objectives like 20x 0.40. So flare from there.

Second guess: the larger aperture means more aberrations for the accessories in the tube. An EPI prism can give a lot of spherical aberrations, but normally we don't see it because the light beam in the tube is very narrow (red), back aperture is small, so aberrations are small. But the 20x 0.75, the light beam is double large than normal, so aberrations are much larger, image may get hazy.
We are picky about glass coverslips, 0.17 mm ok and 0.19mm bad. Similarly, putting a 20mm thick prism afterwards, that may give some problems too.
Infinity system may be a bit better because accessories like the prism won't add much aberrations, at least on axis.

More troubles: in the Nikon 20x 0.75, they put so many big lenses in it, that there is no place for a final baffle, as in most normal objectives. The baffle will restrict stray light. I've noticed that my 20x 0.75 projects a very large cone of light outside its intended field (yellow in the image), much larger than any other objective, apart from simple 4x. This unnecessary light may be reduced with the Kohler field diaphragm - but some will get through anyway, especially with a thick and glittering paper sample. Then, reflections in the tube and accessories will lead some stray light to the sensor.

So, the flare issue probably arise from the large diameter of the back lens.
The small RMS mount screw, and the short 45mm length of the old Nikon CF 160, surely forced the designers to cut corners, remove baffles etc, so to fit more lenses in the small space. But even for the CFI (25mm thread, 60mm length) looks like there isn't still enough space for a proper design; the objective is full of glass up till the brim. Mitutoyo objectives are much bigger - stray light can be controlled more easily, designers are less stressed.

Solutions, against flare, it may need two or more baffles to block all those nasty reflections. For controlling the aberrations, like spherical from the EPI prism, it may even need a baffle right above the objective, to stop it down at a more reasonable NA like 0.50. Baffles are getting popular those days!
Spherical aberration may also be zeroed by altering a littlebit the tube length - so the objective and prism compensate each other.

My 20x 0.75 is currently working with an elegant cardboard baffle glued on the back - but that was done just to increase depth of focus; I had no flare issues (no EPI prism etc). It is a very bad fix, throwing resolution down into the toilet... anyway, the corrected field gets larger, for me was a net improvement. The Nikon 20x planapo is better than anything else I have, even stopped down at NA 0.40.
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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#16 Post by microcosmos » Wed Jul 14, 2021 12:30 pm

Thank you, this is very informative. Yes, my CF N 20x 0.75 has a huge lens right at the back.

Fortunately, even with the design limitations, it still produces really good images under normal circumstances, even with thick samples. It's only when the EPI mirror (in my instrument it's not a prism) and/or analyzer assembly are stacked on that it flares, and then the problem is only really visible with thick samples.

The extension of mechanical tube length beyond 160 mm could have further worsened the overall image. In my microscope, there are no optical elements in the EPI and analyzer assemblies that bring the tube length back to 160 mm, although those would create problems of their own.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#17 Post by apochronaut » Wed Jul 14, 2021 1:13 pm

microcosmos wrote:
Tue Jul 13, 2021 9:38 am

The flare returns if I reinstall either the analyzer assembly or epi-illuminator, but it seems worst when both analyzer assembly and epi-illuminator are in place.
This suggests that spherical aberration may be a partial culprit. Using a tube length that is 40 or 50mm longer than designed for can't be good.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#18 Post by MichaelG. » Wed Jul 14, 2021 1:15 pm

It would be good to see what the objective is really capable of
… I would be tempted to to test it on a much bigger-bore tube
… or maybe even use a box, for a convenient lash-up.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#19 Post by microcosmos » Wed Jul 14, 2021 2:50 pm

apochronaut wrote:
Wed Jul 14, 2021 1:13 pm
microcosmos wrote:
Tue Jul 13, 2021 9:38 am

The flare returns if I reinstall either the analyzer assembly or epi-illuminator, but it seems worst when both analyzer assembly and epi-illuminator are in place.
This suggests that spherical aberration may be a partial culprit. Using a tube length that is 40 or 50mm longer than designed for can't be good.
I should have been more careful about this. I didn't pay attention and assumed that Radical didn't deviate too far from 160 mm but this turns out not to be the case. It is actually the least of the problems with that microscope. For example the instrument did not even have a field diaphragm and I added one myself which made a big difference to the image.
MichaelG. wrote:
Wed Jul 14, 2021 1:15 pm
It would be good to see what the objective is really capable of
… I would be tempted to to test it on a much bigger-bore tube
… or maybe even use a box, for a convenient lash-up.

MichaelG.
I'm getting another microscope in the near future as the Radical has too many design problems that make it very difficult to use for serious work. When that happens I will test it again, although I'm not sure how much wider the bore would be.

How do you test it using a box? How do you align all the optical components?

How about testing it in a darkroom by screwing all the optical components horizontally onto a table like they do in the research labs. The bore would effectively be infinitely large. But I don't have the equipment.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#20 Post by MichaelG. » Wed Jul 14, 2021 5:37 pm

microcosmos wrote:
Wed Jul 14, 2021 2:50 pm

How do you test it using a box? How do you align all the optical components?

How about testing it in a darkroom by screwing all the optical components horizontally onto a table like they do in the research labs. The bore would effectively be infinitely large. But I don't have the equipment.
That’s why I suggested you might use a box
… Two parallel sides … Add components from your Radical ‘scope.
The best way to check alignment is optically; so you only need a way to adjust things
Miracles are worked every day with Gaffer Tape !

MichaelG.
.

Edit: Just to be clear __ I was thinking of a reasonably substantial wooden box, not a cardboard carton.
Think in terms of the old wooden cameras, and you will see where I’m coming from.
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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#21 Post by microcosmos » Thu Jul 15, 2021 5:18 am

MichaelG. wrote:
Wed Jul 14, 2021 5:37 pm
microcosmos wrote:
Wed Jul 14, 2021 2:50 pm

How do you test it using a box? How do you align all the optical components?

How about testing it in a darkroom by screwing all the optical components horizontally onto a table like they do in the research labs. The bore would effectively be infinitely large. But I don't have the equipment.
That’s why I suggested you might use a box
… Two parallel sides … Add components from your Radical ‘scope.
The best way to check alignment is optically; so you only need a way to adjust things
Miracles are worked every day with Gaffer Tape !

MichaelG.
.

Edit: Just to be clear __ I was thinking of a reasonably substantial wooden box, not a cardboard carton.
Think in terms of the old wooden cameras, and you will see where I’m coming from.
I'm not ready to do this at the moment, as I wish to move on and explore sample preparation methods and microscopical techniques. I've spent a month trying to fix this microscope and have learnt a lot so far. I think your suggestion is very interesting and I may get to it in the future when I can allocate more time to the optics side of things.

I think the optical alignment is the difficult part. I don't have the tools to adjust, and I imagine it would be very difficult to align precisely enough by hand, eye and gaffer tape to be able to say from the experiment that any aberration/flare is due to the objective and not to imprecise alignment. The components have to be precisely aligned along both x and y coordinates, plus perpendicular to the optical axis.

On top of that, I have to have some way to focus the specimen.

Gaffer tape would also be prone to creep because of the tension in the tape when it is holding something down that will pull it away from the object it is sticking to, because the tape is not rigid.

Other things also need to be set up properly, such as Köhler illumination and flocking the inside of the box, etc. The question is how I can make the device good and precise enough, with reasonable cost and workload, to obtain sufficiently accurate and reproducible conclusions.

Has anyone on the forum done something like this?

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#22 Post by MichaelG. » Thu Jul 15, 2021 7:01 am

Sorry … I obviously still haven’t explained myself properly

I was effectively proposing that, exclusively for the purposes of checking whether the microscope body tube is the source of your flare problem, you temporarily replace that tube with something much bigger.

The wooden box was the simplest idea I could suggest, for someone with limited workshop facilities
The Gaffer Tape would not be structural, but would be for sealing the light-leaks after you have adjusted the components with screws or packing-pieces.

Perhaps wrongly: I assumed that you would be able to use the mounts and mechanisms of the ‘Radical’

MichaelG.

.
Temporarily substitute a light-tight and blackened box
Temporarily substitute a light-tight and blackened box
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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#23 Post by microcosmos » Thu Jul 15, 2021 8:57 am

Thanks for clarifying, this sounds quite doable actually.

Let me put this project on the back burner until my new microscope arrives and I test the objective on that microscope, which I can also rig with an excessively long tube. That would yield additional clues and depending on the results may make it unnecessary to test with a box.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#24 Post by microcosmos » Sat Jul 17, 2021 7:59 am

Image


I took a photo of the view through the photo tube of the trinocular head. The undersized and scratched (in the factory) half-mirror of the epi-illuminator is scattering a lot of light, and the brightly lit circle is where the badly constructed analyzer assembly is, which reflects a lot of the light coming from the huge aperture of the plan apo 20/0.75.

The rainbows are caused by the camera, not the microscope.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#25 Post by MichaelG. » Sat Jul 17, 2021 8:10 am

Interesting photo !!
Thanks for sharing it.

I really think you should try that Nikon in a stripped-down ‘HotRod’ microscope.
… When it comes to performance “Less is More” :D

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#26 Post by microcosmos » Sat Jul 17, 2021 1:29 pm

No problem. Optics are really fascinating although I'm just a novice.

I did sort of try the Nikon in a kind of stripped-down version of the Radical microscope by connecting the head directly to the turret, which as I mentioned greatly improved the image of the paper. This is now the configuration of that microscope.

What is still in the queue is extending the tube beyond 160 mm but without all the stray light, to see what the objective does.

It's easier for me to find a section of PVC pipe than a box that's rigid enough, but I have to figure out how to saw the pipe exactly perpendicular to the optical axis.

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Re: Strange flare in Nikon CF N plan apo 20/0.75 objective

#27 Post by microcosmos » Fri Jul 23, 2021 7:28 am

I have tried the Nikon CF N Plan Apo 20x/0.75 160/0.17 on an Olympus BHSP. It gives perfect images with transmitted light through thick paper as far as I can see. And I can't see any difference between using Nikon and Olympus eyepieces, although the Olympus WHK eyepieces are supposed to be compensating for their own objectives.

I think it is alright on this microscope because I believe the analyzer assembly has a 1x lens that effectively corrects the tube length to 160 mm although it's physically longer than that.

Furthermore, in the BHSP everything in the optical tube is black, unlike the lots of white surfaces and light-scattering obstructions in the Radical instrument, for which I have given the pun "optical train wreck".

Unfortunately I can't test the objective with an epi-illuminator stacked onto the BHSP, as previously planned, because the epi-illuminator is designed for infinity-corrected objectives and has a tube lens inside it.

I will have to find a chance to test the objective with the wide, empty tube.

An alternative would be to use optical engineering software to ray-trace the objective under various conditions, but we need the files and parameters for the lens elements and the opto-mechanical components.

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