Question on Kohler set up

[pippo1234]

Hi,

I have just acquired a Vickers M15 microscope and I have a doubt concerning setting up Kohler illumination.

I have followed the instructions here https://www.scientifica.uk.com/learning ... lumination or similar ones and set everything up with the 10x objective. I am confused about what I need to adjust when I switch to another objective e.g. 40x. I understood I just had to adjust the aperture of the field diaphragm but not refocus and/or re-centre the condenser. The issue is, if I just adjust the aperture of the field diaphragm when switching to the 40X, the diaphragm does not appear centered as I stop it down. The blades seem to be closing from the bottom of the image rather than concentrically. Could it be due to the fact that the 40x objective is a phase contrast one (and the condenser is not a phase contrast one) while the 10x is not? Or am I missing something more fundamental? The condenser appears properly centred at 10x.

Many thanks in advance.
Giulio

[apochronaut]

The fact that your 40X is a phase objective shouldn't impact your ability to image the field diaphragm leaves. Just make sure that the objective is being used with the open port in the condenser to adjust Koehler , not the phase diaphragm.
If there is a lack of parcentering, obviously there is something causing that. A burr on an objective seat, mis-machining of an objective housing or the nosepiece. Maybe even a bad thread. Check for any nicks or damage first to rule that out.
Your problem is most probably one of poor parcentering between your objectives, caused by a slight machining error. This happens from time to time, moreso with cheap microscopes, which yours is not. Try moving the objectives around the nosepiece one place at a time and check for centering. You can also reverse their order and try in the other direction. With a 4 place nosepiece you have 8 possible relationships beteen the objectives, if you want to keep them in order. 6 with a 3 place.
The objective bores in the nosepiece and the objective barrels themselves will have some variance. This is complicated a bit on the Vickers due to the fact that the 10X is almost full length. In one of objective locations the best case centering will occur. You may even find a place where errors in the nosepiece and barrel cancel and provide almost perfect parcentering.
Centering of the higher magnification objectives is more important than the lower ones. Best to center those and if necessary, have a 10X a little off. You won't need to center it necessarily to focus on the leaves but with a centering condenser, you might as well.

[pippo1234]

Many thanks! That was very helpful and I may have figured out what the issue is. If I carry out the Kohler set up with the 40x or 100x objectives I never get to see the field diaphragm leaves even when fully stopped down. I only see a red shadow from the bottom and a blue one from the top of the image depending on the position of the condenser. I conjecture that proper focus of the condenser with those two objectives is achieved when neither the red or blue shadows are visible. The field diaphragm leaves, though, are never visible. Which would mean that at the end of the Kohler setup the field diaphragm should be fully stopped down with those two objectives. What I referred to as the fields diaphragm leaves closing from the bottom in my original message was actually the red shadow I am referring to above. It goes away if I refocus the condenser.

Incidentally, I have swapped the objectives around but it made no difference which suggests that nothing is wrong with either the objectives or the nosepiece. Phew!

Does this make sense or am I barking at the wrong tree? I am really new to all this, so these are really wild guesses...

[Hobbyst46]

Many thanks! That was very helpful and I may have figured out what the issue is. If I carry out the Kohler set up with the 40x or 100x objectives I never get to see the field diaphragm leaves even when fully stopped down. I only see a red shadow from the bottom and a blue one from the top of the image depending on the position of the condenser. I conjecture that proper focus of the condenser with those two objectives is achieved when neither the red or blue shadows are visible. The field diaphragm leaves, though, are never visible. Which would mean that at the end of the Kohler setup the field diaphragm should be fully stopped down with those two objectives. What I referred to as the fields diaphragm leaves closing from the bottom in my original message was actually the red shadow I am referring to above. It goes away if I refocus the condenser.

Incidentally, I have swapped the objectives around but it made no difference which suggests that nothing is wrong with either the objectives or the nosepiece. Phew!

Does this make sense or am I barking at the wrong tree? I am really new to all this, so these are really wild guesses...

The red and blue hues are probably because the condenser is not achromat.
The field aperture leaves should be visible when it is stopped down (fully closed) even with the 40X and 100X objectives. Have a contrasty specimen on the stage and focus on it, then try to brings the field aperture opening into focus, by moving the condenser up or down. The 100X needs immersion oil to give a sharp view.
If two objectives are not parfocal, switching positions will not change it. If that is the meaning of "swapping around".

[pippo1234]

The field aperture leaves should be visible when it is stopped down (fully closed) even with the 40X and 100X objectives.

They are not as far as I can tell. The red shadow at the bottom does have the shape of the edge of the diaphragm but never goes into focus. It just gets shifted in and out (in the horizontal plane) while the blue one moves in the opposite direction as I adjust the condenser. I take the condenser to be focused when both the red and blue shadows are now longer visible (the light circle appears centered).

If two objectives are not parfocal, switching positions will not change it. If that is the meaning of "swapping around".

I interpreted what apochronaut wrote as having to swap the objectives across different sockets in the nosepiece.

[75RR]

I am confused about what I need to adjust when I switch to another objective e.g. 40x. I understood I just had to adjust the aperture of the field diaphragm but not refocus and/or re-centre the condenser

Every time you change to another objective - you have to set up Köhler again from scratch.

With practice it only takes a few seconds (10-15) and one normally does not change objectives within that time frame too frequently ;)


Here is a link to an excellent tutorial on setting up Köhler:

http://zeiss-campus.magnet.fsu.edu/tuto ... flash.html
.

[pippo1234]

The instructions here http://www.brunelmicroscopes.co.uk/kohler.html prescribe to fully close both the field AND substage condenser iris. All the others I have followed either do not mention what to do with the substage condenser or say to leave it wide open. If I close both I can see the field (I think) condenser leaves even with the 40x objective when the field condenser is fully stopped down.

Furthermore, the M15 has centering screws for the field condenser too. I think they are absent from modern microscopes and all the instructions I have consulted don't mention what to do with them. Experimenting with them a bit makes me think that one can improve things by centering the field condenser too, but I'd like to have a more objective way of going about it.

[75RR]

The instructions here http://www.brunelmicroscopes.co.uk/kohler.html prescribe to fully close both the field AND substage condenser iris. All the others I have followed either do not mention what to do with the substage condenser or say to leave it wide open. If I close both I can see the field (I think) condenser leaves even with the 40x objective when the field condenser is fully stopped down.

Furthermore, the M15 has centering screws for the field condenser too. I think they are absent from modern microscopes and all the instructions I have consulted don't mention what to do with them. Experimenting with them a bit makes me think that one can improve things by centering the field condenser too, but I'd like to have a more objective way of going about it.

Your last post tells me that you have not tried the interactive tutorial yet. Do so, it will clarify the steps for you.

http://zeiss-campus.magnet.fsu.edu/tuto ... flash.html

[pippo1234]

Thanks a lot 75RR! That page has no explanation of the filament alignment (although it was the starting point of the interactive tutorial), hence my missing that step. I have now googled it and finally know what the two adjustment screws at the field converter are for. I have tried to follow the instructions here https://www.olympus-lifescience.com/es/ ... anskohler/ to align the filament, but the lamp has a frosted glass and I cannot see any filament. I have tried to the best of my ability to center the light circle projected onto a sheet of paper held over the field condenser. Is that enough or am I still missing something?

[75RR]

Some microscopes are not setup for Köhler, instead they use critical illumination. This might be the case with yours.

See Chapter 7 of Microscopy Primer by Frithjof A. S. Sterrenburg: http://www.microscopy-uk.org.uk/index.h ... /intro.htm

All of it however is worth a read

[pippo1234]

I guess a couple of pictures are worth a thousand words.

The first one should clarify that the microscope has a field condenser. As for the second one, are you saying that is the wrong bulb for Kohler illumination?

[MichaelG.]

Grab these whilst they are available: https://groups.rifeforum.com/Files/Micr ... formation/

Only just found them ... I will read later.

MichaelG.

[pippo1234]

Grab these whilst they are availabl

You've made my day. Thanks!

[75RR]

Grab these whilst they are available: https://groups.rifeforum.com/Files/Micr ... formation/

Only just found them ... I will read later.

MichaelG.

Nice find!

[pippo1234]

The adjustment procedure is on p. 14, the bulb is displayed on p. 15. Indeed it should not have a frosted glass. Am I right in thinking that this https://www.bltdirect.com/lucas-llb319-6v-18w-ba15d-sbc is the right replacement bulb?

[MichaelG.]

The adjustment procedure is on p. 14, the bulb is displayed on p. 15. Indeed it should not have a frosted glass. Am I right in thinking that this https://www.bltdirect.com/lucas-llb319-6v-18w-ba15d-sbc is the right replacement bulb?

I don’t think so
... the illuminator shown on p15 is the 30w version that fits into the rear of the ‘scope base

Your illuminator appears to be the one described and and illustrated on pp13-14
... it is stated to use a bulb with ‘pearl frosting’ through which the position of the filament can be seen.

MichaelG.

.
P.S. __ It’s not quite Köhler : You might find it useful to read through this thread:
viewtopic.php?f=5&t=9772

[apochronaut]

If the microscope has no mechanism with which to center the filament, then the rest of the illumination system must be centerable against the filament. That is the reference point. Microscopes with centerable filaments do not need a centerable field diaphragm . All microscopes have a centered reference point in the illumination system that everything else is centered to. Some microscopes use bulbs that are very carefully aligned during manufacture, so the degree of filament alignment error is low. That is one of the reasons why certain bulbs are so expensive. Unfortunately there are now many knock offs of formerly precise bulbs that have wonky lopsided filaments or the wrong style of filament in them altogether.
Choosing a quality bulb with original specifications is important.
That bulb in your illuminator looks suspiciously like the 13 watt frosted bulb that was used in the old AO self-illuminated DF condensers. Do you find the illumination a bit dim?
The frosting of the bulb may be critical to providing an even field by that in base illuminator. Sometimes microscope manufacturers contracted to bulb suppliers to make custom bulbs for their application and once the market fell off, production ceased. Hosebuchi still makes an 18 watt ba15d bulb but not frosted.

[MichaelG.]

130CA747-2583-442D-8E09-9D65EBCEA5C7.jpeg (153.11 KiB) Viewed 7360 times

.
MichaelG.

[apochronaut]

So that is a rotational alignment so that the filament is fully perpendicular to the axis of the illumination beam. There is no provision for centering the filament. Presumably a well made bulb would be fairly centered but would still need tweaking. I have seen some bulbs that should have had well centered filaments that were so bad , that they were barely alignable, even with the mirror and bulb socket screwed every which way.

Swift had a frosted bulb with a built in reflector. It can be rotationally aligned without being able to view the filament, simply by aligning the position of the reflector. It is also a ba15d base and 18 watts. It might make a good retrofit, as long as the base to filament distance is the same.
https://www.ebay.ca/i/264728224664?chn= ... gIk9PD_BwE

[MichaelG.]

So that is a rotational alignment so that the filament is fully perpendicular to the axis of the illumination beam. There is no provision for centering the filament. Presumably a well made bulb would be fairly centered but would still need tweaking. […]

.

Yes, that was my reading of it
I don’t think I have ever knowingly seen a Vickers M006035 bulb, so don’t know how good they were.
... they may, or may not, have been ‘selected’

MichaelG.
.

Edit: Here is a view of the more sophisticated illuminator, for comparison:
https://dlib.york.ac.uk/yodl/app/image/ ... ref=search

[pippo1234]

Thanks for the hand holding. There are so many subtleties.

1. It turned out the bulb was turned 90deg. The filament mount was north and south rather than east and west. I have turned it the right way.
2.

There is no provision for centering the filament.

So, if one is able to properly align the bulb rotationally isn't the combination of centrable field diaphragm and centrable and focusable substage condenser effectively equivalent to proper Kohler?
3.

That bulb in your illuminator looks suspiciously like the 13 watt frosted bulb that was used in the old AO self-illuminated DF condensers. Do you find the illumination a bit dim?

Indeed I find it quite dim. Even more given that the eyepieces are not wide field. Is there any reason I could not replace them with a pair of wide field ones? I have just taken delivery of a reticuled WF one (to aid centering) and it makes a world of difference. Do the eyepieces have to match the objectives? If so what do I have to look for?

[apochronaut]

Yes. Since the lamp filament will be close to centered but not have a mechanism to preciely center it, the mirror and field diaphragm are used to bring a centered beam to the condenser. The condenser is then centered to suit.
Presumably the nosepiece and binocular head are in line.
No reason you should not be able to set up Köeler or something so close as to be indistinguishable. If you can image the subject and field diaphragm leaves in focus at the same time and get the diaphragm aperture and condenser aperture both centered you will have a good illumination system.

Vickers had some w.f. eyepieces. They used some Kellners that had an 18mm f.o.v. and had a very nice 18mm f.o.v. w.f. compens 10 X. I use a set of those with Bausch & Lomb apochromats. Nice to get an 18mm f.o.v. with those.

I assume your objectives are those fairly ubiquitous Microplans? I can test out some options for you and report back. I have retained a lot of eyepiece samples, for just such planarity matching.

[pippo1234]

No reason you should not be able to set up Köeler or something so close as to be indistinguishable. If you can image the subject and field diaphragm leaves in focus at the same time and get the diaphragm aperture and condenser aperture both centered you will have a good illumination system.

After figuring out that I had to center the field diaphragm everything seems spot on and well aligned. Even the re-adjustment when switching objective is minimal.

I assume your objectives are those fairly ubiquitous Microplans?

I assume so. They are
- 3x, 0.1 (T9218)
- 10x, 0.25 (T6086)
- 40x, 0.7 +Ph.2 (V1102)
- 100x (oil),1.3 +Ph.3 (T7917)

Judging from the centering eyepiece which is 18mm, 18mm would be a blessing.

[apochronaut]

If they are Microplans, it is marked on the barrel. If Plan, that then. I used a couple of Cooke-Baker( later they were branded Vickers) Microplans and checked out about 20 eyepieces.About 5 AO with between 19 and 20mm fields, 3 B & L of the same , a couple of Reichert at 18, couple of C.B.S. at 18, Zeiss KPL , Leitz periplan and Olympus.

I used a set of Vickers W.F.. 10X compens as a benchmark. Normally, compens eyepieces under correct with achromats. While these ones are very good up to about 16mm with the Microplans, enough ca banding becomes apparent outside of that and planarity falls off.
Most of the rest of the eyepieces were at least as bad with a few slightly better. The Zeiss 12.5X KPL and CBS 10X PK were about the same, with only slight ca at the periphery but each is working at a slightly restricted field, maybe 16 or 17mm, which any of the others with 20mm fields achieved slso. I got the impression that a Zeiss 10X KPL for instance , which would provide a wider field would show lateral ca outside the limited field of the 12.5X.
The best, even though it doesn't provide a perfectly plan field and has a slight amount of peripheral lateral ca is an Olympus 10X WF Bi. The field is probably 20mm. It has a raised collar above the eyelens for glasses on use. i think that eyepiece was for the short barrel objectives. The Olympus WK 20X L was not quite as good but close.

You may have standard achromats if your objectives are not marked Microplan. The corrections would likely be similar but the achromats would suffer from a lower level of planarity. I believe the Microplans are supposed to be semi-plan. The results with the Olympus eyepieces would be similar with the achromats, just greater lateral ca and less planarity . I can imagine a great improvement over a standard pair of Huygens. Certainly the image the Microplans give is excellent at a 20mm f.o.v.

[pippo1234]

Thanks a lot for your time and help! From reading the manual, it seems they are run-of-the-mill achromats. I will look out for Olympus WF10x. Most of them seem to be in Japan or the US right now.

[apochronaut]

I would stick to the same model. Over time , Olympus has transferred more of the needed corrections to other optics upstream and the newer eyepieces have less correction. Your objectives, designed in the 60's/70's, probably just need similar corrections as the short barrel Olympus objectives of the period.
here is a picture. If I had a pair, I would offer them to you but I only have 1. Still available though, if you want it. It's in good shape. You probably can find a mate in the U.K.
I'll post a photo in a short while, so you know what they look like.