How to set tube length on the fixed (without diopter adjustment) side of the Reichert 410 Microstar IV binocular head?

[hans]

This question is partly practical (although I suspect the practical effect fairly subtle) and partly out of theoretical curiosity, wondering how it originally would have been set at the factory.

The wide, shallow groove near the end is where the set screws in the outer tube part (which the eyepiece fits into) clamp. The groove allows the position of the outer eye tube (and therefore eyepiece) to be adjusted with a total range around 4 mm relative to the telelens and color-correcting doublet before clamping it in place with the set screws.

I think the basic question here is, what apparent distance is the virtual image from the eyepieces intended to be projected from? If that is known then I am thinking the eye tube position could be set like this:

1. Manually focus a camera on an object at the intended virtual image distance.
2. With the head removed from the stand point the telelens at a distant object.
3. Set tube length by focusing the distant object with the camera looking through the eyepiece in live view mode.

One guess is that the virtual image from the eyepieces should be focused at infinity. A while back I tried accurately setting infinity-focus from the eyepieces using a camera but found that it was not the most comfortable focus distance, my eyes preferred a somewhat closer focus. This is undoubtedly somewhat person-specific, and perhaps I am slightly nearsighted, but presumably microscope manufacturers would design around some typical most-comfortable focus distance, not necessarily infinity?

Another guess is that the distance should be consistent with what is implied by the inward angle of the eye tubes.

Does anyone know of a microscope where there is an adjustment like this and also an official service manual available that covers how to set it?

Finally, in the case of the 410, it is not obvious what the main effect of changing this should be -- spherical aberration due to changing the objective working distance? Or lateral CA due to changing the distance from the color-correcting doublet to the intermediate image? Or perhaps both are significant. If other microscopes do not have a similar adjustment that would point to it being related to the doublet.

[hans]

There is a similar procedure in the Olympus BH2 (BHS) Repair Manual section 11-5 "Adjustment of mechanical tube length" but not a clear description of special tools C-15 "Focusing magnifier (PM-FT-36)", KN0007 "Standard objective for optical tube length alignment", and KN0022 "Special WF 10X for optical tube length alignment".

Does anyone know if C-15 similar to a standard phase telescope? I would guess the purpose of using a "focusing magnifier" is to restrict accommodation range and minimize variability due to differences in eyesight among technicians? In any case it seems like a camera manually focused at 1,000 mm would be a suitable substitute.

I am guessing "standard" referring to KN0007 does not mean an objective that would be used normally, but rather some special objective that has a reticle/target permanently fixed to it at the correct working distance? So, "standard" as in it serves as a standard against which tube length is set? Otherwise the procedure does not make much sense. This aspect seems easier in the case of an infinity-corrected system like the 410 as infinity "standards" are readily available in any room with windows.

There is a drawing of KN0022 shortly after in section 11-6-6. It sounds like this is probably just a focusing eyepiece, but with calibrated scale used to determine whether tube length is meeting specifications?

[hans]

I did a bit more careful measurement to get an idea what the adjustment corresponds to in terms of virtual image distance. Head is removed from the microscope, laying on its side, with telelens pointing out the window. With the set screws loosened slightly the total range of the tube length adjustment (limited by the width of the groove) is 2.8 mm. At minimum tube length (eye tube pushed as far as it will go onto the head) the image is in focus with the camera lens focused at 50 cm, at the midpoint of the adjustment range the image is at approximately 2 m, and at maximum tube length the image is a little past infinity. (My camera lens, which is able to focus slightly past infinity, can almost but not quite focus.)

50 cm is a little closer than I find comfortable. Rough measurement of the inward angle of the eye tubes of the in the previously linked thread implies something like 1-2 m equivalent viewing distance, fairly close to the image distance at adjustment midpoint. Not sure how much variation there might be between heads (presumably this is why it is adjustable in the first place?) but in the absence of other information about how this is supposed to be set, it seems reasonable to either leave it at the center of the adjustment range (lifted 1.4 mm from the fully seated position) or put the virtual image at 1-2 m distance using a manually-focused camera, analogous to the Olympus BH2 procedure.