A view into a Wild M20 binocular head: a discussion about its design and some delamination effects

That made me notice that indeed the interpupillary distance was sent on the M20 by sliding the left eyetube, while the right eyetube was unaffected. This is quite unfamiliar, most binocular heads compensate for the interpupillary distance by varying symmetrically both eyetubes. How is it possible to maintain focusing by the objective in the same plane respective to the oculars, while the objective-ocular distance is varied? How is this specially patented automatic balance performed ?

I immediately wanted to know more about this (as you're reading this, you already know my geekiness is clearly above average). My first attempt was to browse the web. Although the M20 is a famous, well-documented stand, I couldn't find anything about the binocular tube internal construction. Of course then, the natural idea is to go by yourself and unmount the item. However, that goes against one of the major rules in microscopy : "if it ain't broken, then don't fix it". I don't consider unmounting an optical piece that doesn't have any trouble is a good idea. Chances are high, either to bring some dirt or (much worse) to misalign some parts while remounting.

Finally, I purchased a second M20 stand (because I wanted its Fluotar objectives and wide-field eyepieces). The binocular head of this second M20 had some severe-looking troubles: anti-reflective coatings were attacked, and the optics of the left eyetube were showing some signs of delamination. So I came out with an optically imperfect bino, that was enough to clear-off the aforementioned rule. Hereafter, I will thus discuss about the internal optics of this defective bino' head, and check about possible implications towards degrading image quality.

Internal optics - automatic balance revealed

Unscrewing the binocular head is quite straightforward. The first optical element is a prism to allow for inclined vision, and is quite usual in microscope tubes.

The main optical element is depicted in the figure below. I hope my annotations will make the optical path easy to understand for everybody. So here is the secret into the so-called automatic balance: a diverging lens fixed to the 50/50 beamsplitter is followed by a converging lens (almost same curvature). Varying the distance between these lenses compensates the longer optical length by refocusing the image formed by the objective. This is a clever construction, yet I consider the prism-based system (Zeiss) or the symmetrical translation (Leitz) are intrinsically better. Maybe Wild was coerced into that design because of patent issues?

How far does delamination in the binocular head influence image quality ?

Looking at the internal state of the optics of the left eyetube, some troubles are apparent: the diverging lens glue to the prism is strongly delaminating (image below left), while the anti-reflective coatings also show signs of wear (image below right, these are not due to bad cleaning procedures, it's rather the coating that is peeling off).

Can this be fixed ? I'm afraid the answer is no. For the anti-reflective coatings, one feasible solution would be to remove the coatings, but it would be hard not to scratch the glass without access to ultra-strong (hydrofluoric) acids. For the delaminating lens, even if the lens and the glue can be removed, I don't see how it would be possible without specialised equipment to (i) avoid any air bubble while gluing again the lens and (ii) centering all the optics. So another rule applies here: "if you don't know reasonably how to fix it, then don't go any further".

The second question now comes to whether all this really affects imaging, and to what extent? To check this, I used a Wild Fluotar 40x/0.75 phase objective, and my modified Brunel DCM130 USB camera (I removed all projection optics in the DCM130 to keep only the CMOS chip, that is directly inserted in a image plane by the objective. Strictly speaking, the tube length here is close to 180mm, and doesn't meet the specifications for the objective, but as all this is futile anyway, I don't really care.) I'm not sure this objective choice is the best, I wanted to take a high-NA objective that would be more sensitive to additional aberrations.

The figure below illustrates the different tests undertaken. The sample is from Klaus Kemp's excellent 8-form diatoms test slide. The images have been resized, and contrast slightly modified to better illustrate the discussion.

Conditions:
A- bright field, incoherent illumination, aperture iris open to 100% of objective NA.
B- bright field, coherent illumination, aperture iris to 5% of objective NA.
C- bright field, partially coherent illumination, aperture iris open to about 15% of objective NA.
D- Phase contrast, annular ring for 40x.
E- Annular oblique illumination, incidence from the top.
F- Dark field, this one is taken with Wild Fluotar 20x/0.60.

The effect of delamination in the eyetube is only to be seen in case B, and to a minor extend in case D. It corresponds on the darker zones on the left of the images B,D. In that cases, resolution is not noticeably affected, it is mostly the background that turns inhomogeneous.

Conclusions

Delamination in the internal eyetube is shown to have no noticeable effect on most illumination conditions. To see any effect, the illumination has to be turned into strong coherence (small angular divergence of the light illuminating the sample). In that latter case, it is mostly the background that turns inhomogeneous. This isn't very detrimental for digital imaging, as a background can always be subtracted. Fortunately, there is a right eyetube that does not have these supplementary lenses !

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