How to adjust Reichert 410 nosepiece detent angle?

[hans]

I received a calibration slide/stage micrometer today and noticed an asymmetry in the chromatic aberrations that is common to all objectives. I believe the problem is that I didn't get the detent spring, which I removed when I disassembled and cleaned everything, back in the correct place. Here is a photo of the nosepiece from another 410 that I haven't put back together yet:

reichert-410-nosepiece-1024.jpg (202.64 KiB) Viewed 2852 times

I realized removing it might be a dumb thing to do shortly after loosening the screws, but then thought maybe it would not need precise setting since the position of the objective can also be adjusted via the centering screws holding the nosepiece to the focusing arm. Unfortunately it looks like the asymmetry in the CA is mainly a function of the angle of the objective relative to the head rather than the position and if the detent is not in the right place the objective ends up tilted relative to the optical axis.

It seems like the adjustment could be done either optically, by rotating the nosepiece while observing the CA then drawing a match mark across the nosepiece joint to use after disassembly, or mechanically, by setting the barrel of the objective perpendicular to the face at the top of the nosepiece that mates with the bottom on the focusing arm. Any suggestions on how to proceed?

[PeteM]

There is likely a bit of slop (oversize holes) between the screws and spring. You could adjust that. However, I'd also check the centering of the entire nosepiece (3 setscrews). Start by visually centering when looking down through the stand with the head off.

[hans]

Pete, sorry, I probably should have been more specific, but I think maybe you didn't notice the additional text below the photo? The holes in the spring are indeed oversize, and the positioning of the spring within that slop is the "adjustment" I was referring to.

[hans]

It just occurred to check another possible source of misalignment which is the mounting of the head housing to the base plate (the part of the head with the dovetail) where there is also quite a bit of slop due to oversize screw holes. Alignment there is affecting alignment of the beam splitter and eyepieces relative to the telan lens and prism. I loosened the screws and reinstalled the head but movement there, as with the nosepiece centering, has small, if any effect on the apparent optical center of the CA in comparison to the relatively large offset I am seeing.

In retrospect, disassembling and dealigning almost every part of the optical path at the same time was probably not the best idea, but a good learning exercise in how to align a microscope from scratch, I guess...

[apochronaut]

I guess you suspect your alignment problem is taking place in the Z axis. The peripheral area of any of the objective's image circle is assymetrically displaying chroma.

The best method to check and see if the problem is due to the active objective port being tipped is to get all the other adjustments into close alignment with your eye. Align the dovetail in the head by eye, the faceplate by eye and the nosepiece dovetail too. All of those components can only be adjusted in the xy plane. Getting them eyeballed into place should suffice . I usually take the condenser out and looking down through the parts, do a rough centering. Refinements can be made by looking down through the optical tube with an eyepiece out . Once you feel confident everything is pretty close you can check the nosepiece.
One thing to watch out for is that if the components being aligned orbitally with 3 screws have had those screws tightened a lot over the years, those screws can create a relief where they seat. Shifting the position slightly can cause one or more of the screws to ride up on the periphery of the seat and lift one corner of the adjustment triangle, causing a Z axis mis-aligment. Just a warning to watch that everything is sitting in it's required plane.

So install the 4 or 10 and rack it down to contact a flat part of the stage. Shining a bright light between it and the stage should show if there is sufficient difference between the two sides. Feeler guages can be used too. You can move the objective until it is level, get an idea how much to move the detent spring, adjust that and try again. You may be able to reach up through one of the objective ports to refine the adjustment with an L screwdriver .

Once you feel you are making headway, the various adjustments can be refined .

If the chroma is really obvious , you might consider looking and seeing if there is a shifted prism? I suppose it could always be caused by a combination of factors.

[hans]

If by "alignment problem is taking place in the Z axis" you mean a tilt of a vertically-mounted component away from the vertical axis of the microscope then yes, I am still thinking that is the case. I had been using the term optical axis rather than Z or vertical to avoid confusion since the light path is not vertical after the head prism. Last night I used a micrometer to select a cover glass close to 170 um thick (measured ~180 um) and put it on the calibration slide with a drop of light instrument oil (still have not got around to buying immersion oil) to get the clearest test image possible. I spent a long time staring at the calibration slide with various combinations of objectives and heads while loosening and moving different parts around. Overall, I am more confused than before but one thing I think is still pretty clear is that the purely planar XY adjustments that don't tip of tilt anything relative to the optical axis (nosepiece centering, for example) tend to shift the field and CA by roughly the same amount and the range of shift that is possible is a small fraction of the diameter of the field. Tipping/tilting components, on the other hand, can produce large shifts in CA with little or no shift in the field depending on the point about which the component is rotated. (I believe that is all expected theoretically, and the point about which rotation of a component would shift the CA without shifting the field is one of the nodal points?)

One thing I noticed last night is that there is a difference in alignment between the two heads I have been using that cannot be explained by mounting slop base-to-housing. One head is binocular which I have been using for a while and as far as I know had nothing wrong with it. The other head is the trinocular one I mention recently that came with a Reichert-Jung 150 and had an alignment problem between the two eyepieces. I have since fixed the binocular alignment problem -- looked like there had been a hit from a hard corner of something at the joint between the tube that receives the eyepiece and the splitter/prism assembly. The hit shifted the tube and also left a nick and raised a bit of metal that was preventing the base of the tube from sitting flat against the face it mates with. The difference in overall alignment relative to the other head makes me worry there is other damage but I can't see anything wrong with the linear bearing slide that allows the prism to be moved out of the way and all the other elements are cemented in place with no sign of having shifted.

Since I have three other binocular heads laying around, I think the best thing to do next, before I try out your advice, is clean those up and decide whether the difference I am seeing between the two I am using is normal variation or damage. I did already eyeball the alignment of everything as best I could when I initially reassembled the microscope.

[apochronaut]

I wasn't thinking in terms of damage, since it wasn't initially mentioned but that is most likely the concern. Perhaps you can build up the trinocular from a binocular. They have mostly universal parts.

[hans]

The difference between the two heads is smaller than the CA offset I originally noticed, and I am still thinking that the position of that detent is the main issue, just trying to make sure I don't end up with two different misalignments cancelling to some degree but still leaving a degraded image.

Regarding binoc/trinoc interchangeability, I did verify that the whole splitter/eyepiece assembly can be swapped, but I have not tried to determine whether the difference between the two heads is mainly in the telan/prism section or the splitter/eyepiece section. (If there is a problem in the telan/prism section of my trinoc then I am probably out of luck for a simple part-swapping fix.) I plan to do that experiment once I have the other heads cleaned up. I also worry that with the heads, since there are some degrees of freedom between the parts that are not adjustable, they may have done alignment of the entire head at the same time during manufacturing. If so swapping parts may not be possible without introducing additional misalignments, or at least may require some shimming.

The situation with the alignment within the heads is further confused by the heads being rotatable, which brings up another question: Is it expected that the microscope would be designed to achieve optimal images with the head in any orientation? Or is the intent that you would normally only use it with the head facing directly forward or backward? (I have found it useful to turn the head 90 deg to the side, since that is the only way my daughter can reach the controls while looking through the eyepieces.)

[hans]

Sorry, no real satisfying conclusion to all this, but I think the alignment is ok now. Thanks for all the suggestions.

The good news is that after cleaning up and testing all the other heads and objectives from the well-used 410s I have been collecting there are no obvious outliers in terms of alignment except one bad 10X objective. The internal alignment of my trinocular head, after fixing the obvious binocular offset, appears to be well within normal manufacturing variation.

As far as the offset CA, I think I was fooling myself by not paying careful attention to my head position. (Talking about my head now, not the microscope's.) I eventually noticed that, while staying fixated on a point toward the edge of the field, head motion within the range possible without significant vignetting has a large effect on the visible CA. I think the initial impression of the CA being offset was just a consistent bias in how I was holding my head due to the workbench/microscope being too low. Then all the troubleshooting trying to look through the eyepieces while moving things around was confounded because tracking motion through the eyepieces typically results in a change in head position at the same time. In other words, I think the shifts in CA I was attributing to moving/tilting various components were actually mostly due to subconscious head movements as I tracked the motion of the field. There is certainly some noticeable CA shift due to these adjustments but not as dramatic as I was thinking and I didn't actually end up changing anything significantly relative to what I had arrived at by eyeballing during the initial assembly.

Now when evaluating alignment visually I am trying to position my head in a more deliberate, conscious manner and getting less confusing results:

1. Fixate on point of interest, then while maintaining fixation:
2. Change body and head positions to get head and eye angles close to neutral.
3. Carefully adjust head position laterally and make a judgement in the position where CA is minimized.