Advice please: High NA air objective with small WD

[Chris S.]

Seeking advice, please.

I've been offered an apo, high NA, air objective designed for no cover slip, that has a tiny working distance of 1.3mm. This objective has a wide front end that will make it nearly impossible to shine reflected light onto the subject. So in my view, subjects for this lens must either be thin and entirely backlit, or be illuminated by light shining through the lens. ("Axial illumination?" I'll use this term, but welcome correction if wrong.)

To use this lens on my current macro rig (the Bratcam), I would need to add a beamsplitter, light source from the side, and perhaps a polarization arrangement to stop light from bouncing off the interior surface of the objective. Pretty sure I could build this, but wish to do so only if time and cost prove worthwhile by permitting photographs that can not be obtained by simpler methods, and show uniquely informed insights.

I also have a Mitutoyo FS-70 head that I've been meaning to integrate into a working vertical rig. This head includes fiber optic axial illumination, which might or might not work well with this objective.

Can anyone with knowledge of axial illumination (if I'm using this term correctly) help me gain a sense of how much additional information, if any, this lens, plus system modifications needed to use it, would offer? Would any additional visual information be worth the trouble and cost? Axial illumination looks, to my uneducated eye, like a microscopic mug shot; are there ways to make this lighting more illustrative?

In advance, thanks. This lens is significantly discounted against list price for a new specimen; but the discounted price is still a lot of money--too much for me unless the lens lets me share significant new information in our forum.

Cheers,

--Chris S.

[enricosavazzi]

I have no specific knowledge to offer, but I have a Mitutoyo FS-60 microscope equipped with four Mitutoyo M Plan Apos, and I rarely use the built-in axial illumination because it gives a very flat and very low contrast "rabbit-in-the-headlights" rendering of almost all subjects. It is especially hopeless on flat reflective subjects.

That said, a high-NA objective should be able to illuminate the subject with a broad cone of light converging onto the subject, while my M Plan Apos have at most an NA of 0.42 and therefore produce an illumination cone with a rather acute apex angle. This might make a difference, at least with some subjects.

The beamsplitter in the FS scopes is extremely thin, and likely so fragile that it is virtually impossible to clean, except perhaps by removing it and immersing it carefully into a cleaning solution.
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--ES

[TheLostVertex]

[Chris S.]

Enricco,

Thanks for sharing your experience with the FS-60 microscope and its built-in axial illumination. “Flat and very low contrast ‘rabbit-in-the-headlights’ rendering” is exactly what I was afraid of. I certainly don’t want it.

The NA on this objective is 0.90, so the illumination cone should have an obtuse angle (128 degrees?), but I don’t think I’ll risk the significant cost of this objective in hopes of using the wide entrance cone to create better modeling.

Steven,

Thanks for the suggestion of a co-axial ringlight. The front of this objective has a diameter of over 32mm, though—so in this case, the approach would create extreme side-lighting.

I’m going to pass on this objective. The $950 USD offer is a big discount compared with the new price of $6300, but is too much for the limited use I can envision for the thing. The lens in question is a Mitutoyo HR 100x/0.90.

Gentlemen, your experience and thoughts have been very helpful.

Cheers,

--Chris S.

[Scarodactyl]

That's certainly a rare objective on the used market. I have found 1ish mm fine for oblique lighting with a fiber optic illuminator, though obviously not as easy as a more reasonable wd. That is at much lower mag though, with subjects that are more forgiving.

[Macro_Cosmos]

I do have a beamsplitter cube coming which I intend to use for diffuse axial illumination.
Similar to transillumination/backlit, axial illumination/coaxial light works too. Personal preference thing I guess, or I may be wrong.

From my experience, I dislike this kind of setup a lot. The light lacks any kind of control, images all almost looks the same and flat. One would also lose a sizable percentage of light, 50% in my case. Open to change my views though, hope the incoming one does better than the ones I use at work.

I've also encountered bright hotspot issues due to the objective lens I use at work. It's not a microscope objective. The hotspot can be removed with polarisers, but images turn to mush, too much for that low MP camera.

I hope it works out in my favour for photography though, the morpho didius butterfly and wafers work incredibly well with epi-illumination.

I suggest cube beamsplitters, they are true beamsplitters.
Info here: https://www.edmundoptics.de/knowledge-center/application-notes/optics/what-are-beamsplitters/

You'll have 50% of light that comes out of one end which is wasted. You must have a way to deal with it. I use a flocked tube that's blocked off at the end.

Several forum users have seen success with reflected light for 1-2mm WD objectives. It will help if you can somehow get rid of the front housing.
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[rjlittlefield]

In principle, I think it should be possible to structure the illumination beam so as to provide asymmetric lighting for modeling, by sending light through only a part of the rear aperture. But I have not tried engineering such a beast, so I have no good idea how difficult that would be. (Murphy's Law predicts it's pretty hard.)

--Rik

[Chris S.]

[Macro_Cosmos]

[Pau]

There are some approaches to avoid on axis epiillumination like the Zeiss h-Pr illuminator which I only know from reading
at Cebulla, W. Handbook of Incident Light Microscopy (Zeiss), pg. 11-12


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Pau