Revolving nose piece

[Frankiev]

Is it possible for the nose piece, and therefore the objectives to slip out of alignment with the rest of the optics. My microscope is a Leitz Wetzlar Dialux?

[apochronaut]

Yes. The nosepiece has to be adjusted so that the axis down through the objective lens is in line with condenser and illuminator axes as well as the observation tube(s)
There are two ways it can be out. One is that it can be off center orbitally or two, the rotary stop can be laterally mis-aligned so that the objective is slightly shifted left or right of center. Usually, the nosepiece is orbitally centered first, followed by lateral alignment.

[Frankiev]

Thank you apochronaut. When I sort this out I'll have rebuilt the microscope !!

[Alexander]

There is always a deviation from perfect centering. That's why petrographic scopes have centering features for every single component.

[Sure Squintsalot]

To clarify: petrographic microscopes have x-y alignment adjusters only for the objectives, critical for precise concerted alignment of rotating stage, Betrand/Amici lens, and z axis of a crystal, commonly used for mineral ID and and limited fabric orientation.

Otherwise, conventional viewing allows for a lot of slop in component alignment.

[apochronaut]

Nosepiece alignment is different than objective parcentering or the alignment of each objective to one another. Usually a nosepiece is centered in the factory and only becomes decentered through some misshap or maladjustment .
I am wondering why you are concerned about this? Each objective is a unique machined part and they can be a little off center. It wouldn't normally be an issue with a Leitz but it can be a big issue with cheap microscopes or cheap objectives.

[Frankiev]

My concern arises from being unable to achieve Kohler illumination. I'm finding it impossible to centre the field diaphragm with the condenser centering screws. There's always a a dark crescent.

[Alexander]

I'm finding it impossible to centre the field diaphragm with the condenser centering screws. There's always a a dark crescent.

In this case I recommend the following:

- open the field iris
- remove the condenser

Is the illuminated field still restricted?

If yes, the light pass is obstructed. Remove any filter slider in the light pass and make sure the trinocular switch is set properly. In case this does not help, turn the nose-piece slowly and watch what happens.

Is the field of view clean without the condenser, the problem may be in the condenser. Place it back and check with and without the top-lens. A de-centered top-lens is a possible source of problem as is a condenser out of axis. Put the top-lens in, use a low power objective like a 2.5x for 4x and check if the light spot is in the exact center of the image.

Remember that Kohler illumination is only possible with the top-lens in place.

Hope, that helps to identify the source of problem.

[Frankiev]

Thanks Alexander. Yes that was very helpful and I have achieved Kohler pretty well.....not perfectly because I still can't get exact centering. On looking at the inside of the nose piece it's clear that someone has been trying to adjust the spring mechanism that clicks the revolving nose piece into place. I wasn't aware that I'd changed anything but when I reassembled it there was an improvement so I shall leave well alone.