Through the microscope

Posts tagged ‘koehler’

The Meopta continued – Kohler illumination shouldn’t be this difficult

My arguments with the meopta microscope continue. I have been trying to get Kohler illumination set up on my Meopta microscope. So far I have had serious problems. I’ve had lovely dark ground images that I don’t want but everything is so squiffy that I can hardly get any light through the thing at all.

After more fiddling and cursing I can announce that I am winning, but I have not yet won the battle. I had to consult the oracle (Merv) and he informed me that there ARE centring screws on the condenser holder. They are very tiny, only about 1mm across and they’re countersunk. You can’t look through the microscope while adjusting a screw that you can hardly see. It’s also impossible to get a screwdriver into the screw with the condenser in place. I loosened the screws off and tried inserting the condenser again but it made no obvious difference so I tightened them up again before they fell out and were lost forever. Bah humbug to the condenser centering hypothesis.

I returned to my first hypothesis (which the oracle confirmed as a good possibility) – the objective lens is not in the right place. When I use a phase telescope it looks wrong. It’s hard to tell what is wrong it just looks off, I can see a bead of light as if it is reflecting on the tip of the lens. I gave the objective a gentle tug towards me and BINGO! We have light! Lots of light! I had to turn the lamp intensity down because my eyes were hurting.

The objective turret IS off. Again, there is no obvious way to centre the objective turret but unscrewing the monocular tube and screwing it back again seemed to fix it. Didn’t fix anything the last time I tried but such is life.

Now I can see that the condenser is pretty much centred. Not perfect but almost there. Hurrah! The condenser iris is almost central when I open and close it. I shall deal with that later. I suspect it is perfectly central and that I need to do a bit more to the objective turret than give it a tug.

So, we have light, what next? Kohler.

I set up my external illuminator as follows:

1) Hold a piece of paper about 30cm in front of illuminator.

2) adjust condenser lens so that filament is seen to be in focus on the paper.

3) open and close field diaphragm to check that light is shining around filament symmetrically

4) Having ascertained that the illuminator is doing everything it should be, move it about 15cms from the microscope and adjust the position of the lamp (without altering the lamp’s condenser position) so that it is aimed at the microscope mirror. Close down field iris.

5) Check the light is hitting the centre of the mirror then tilt the mirror so that the light shines up through the condenser.

6) place high contrast specimen on stage and with condenser racked up focus on specimen.

7) close down condenser iris diaphragm and move the condenser up and down until you see a sharp image of the field diaphragm on the specimen. Open and close the field diaphragm a bit to check you are seeing the field diaphragm and not the condenser iris,

Here’s where I run into problems – the field diaphragm is only just visible. It is very faint, it’s a long way from the clear sharp image I need. It can only just be seen if the condenser is racked right up and even more strangely, it is not circular. it appears to be oval in shape.

Something is rotten in the state of Denmark, as Shakespeare said.  More precisely, something is rotten in Czechoslovakia (for that is where Meopta microscopes were made) and something is rotten in the UK where I am desperately trying to set up Kohler.

It could be the the objective turret again. I’ve improved the situation but I have not perfected it. A misaligned objective could be making the field diaphragm appear oval by deforming the light beam, so to speak. Note how I carefully explain the physics there? 😉

Also,  I’m not entirely sure whether the light hitting the mirror is supposed to fill the mirror or the central portion of it and worst of all  I’m not sure why I don’t know this…. Oh, hold on, yes I do! I’ve never set up Kohler properly with an external illuminator before. All my phase microscopes have built in lights and I’ve used critical illumination much of the time with my other microscopes; that or I’ve used diffusers.  I do not have the image of the filament filling the mirror. I may have to play with this. Perhaps my beam of light is too narrow when it hits the condenser lens. This should be simple enough to test. I just need to move the lamp closer or farther away.

I have to get this working, there is no point attempting phase contrast without Kohler illumination, it won’t work.

I have a couple of other problems which are easier to deal with. The screw that holds the condenser in place is bent and it is hard to tighten when the anoptral phase condenser is in place because it’s too short. I can only just grasp it when the anoptral condenser is in place. This means the condenser is wobbly. If I try to turn the condenser from its bright field position to the 10X phase plate the whole unit moves. If I try to insert the phase wrenches the whole unit moves. This is far from ideal. Fortunately this can be fixed. I have ordered some very long M2 screws which should fit nicely and allow me to tighten the thing up enough.

Also, the Meopta filter holder is in the way. I can’t insert the phase wrench on the right hand side without turning the condenser turret. I won’t be able to do this when the thing is properly tightened up.  I need to remove the filter holder.

There are an awful lot of articles out there about how to set up Kohler illumination. There are very few trouble shooting articles that go beyond “centre the condenser”. Nothing much deals with “this baby is so far out I can’t even see the things I need to see to ascertain what is out of alignment”.

Oh well, at least I can see the field diaphragm now, even if it’s gone pear shaped. That was a joke: it’s actually gone egg shaped.

I have never had so much trouble with a microscope!

On the plus side I’m learning a great deal about fiddling with microscopes. All my other microscopes have been easy to use, they have needed a little bit of centring or a small tweak. I have never had to deal with something that is so far out of whack I don’t know where to begin. I shall have a much better understanding of light and optics and how to tinker when I’m finished.

I have a busy weekend ahead so no time for microscopes. I shall have another bash at it next week. If it continues to frustrate me the bashing may not be metaphorical.

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