Cannot Obtain Köhler Illumination with Olympus BH2 BHT Microscope

[johntb]

I have been unable to achieve Köhler illumination with my Olympus BH2 BHT microscope because I cannot adjust the field iris diaphragm properly. When using the 4x objective, I close the field iris diaphragm all the way and the field iris diaphragm image takes up most of the field of view. I can center it with the two condenser centering knobs, but the edges remain blurry no matter what height I set the condenser at. Furthermore, with the 10x objective I cannot even see the field iris diaphragm image, even with the field iris diaphragm closed as far as possible. Closing the field iris diaphragm with the 10x objective just decreases the light intensity across the entire field of view.

I opened the microscope up and looked at the field diaphragm unit. It appears to be functioning correctly. Fully closed, the field iris diaphragm aperture diameter is roughly 1.4mm. I'm not sure how small it should be when closed though.

The repair manual recommends using centering adapter (B2KC1003) and a special centering telescope (KN0029) to align the field iris diaphragm aperture with the center of the optical axis, but I don't have either of those tools and it doesn't look like I can purchase them anywhere.

The exact configuration I'm using is the Olympus BH2 BHT with the BH2-UCD universal condenser (set to an empty turret slot; the swing-in lens is not used) and an BH2-NA Nomarski intermediate tube (with the slider out, not used). The objectives I'm using are the Olympus DPlan 4 and the Olympus SPlan 10, but I do have higher magnification objectives as well.

If anyone has ideas for how to fix this, please share.

[75RR]

Don't think one can achieve Köhler with very low magnification objectives - you should be ok on 10x and up though.

The exact configuration I'm using is the Olympus BH2 BHT with the BH2-UCD universal condenser (set to an empty turret slot...

Remember that you need to focus on a subject (using the brightfield port) as the first step of the process.

Here is a tutorial that I find is both useful and fun.
Note: in the tutorial you need to begin by aligning the filament, something that you normally do just once (when you set up the illumination).

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

[MicroBob]

What type of condenser do you have and is it complete with top lens?
A simple Abbe condenser offers useful illumination but a less than perfect image of the field aperture. Have you tried it with a 40:1 objective?
The condenser top lens should sit just below the slide.
For low magnifications usually some kind of trick is applied like removing the top lens or lowering the condenser to acheive illumination of the whole field of view. A true Köhler-illumination won't be possible then.
By the way, August Köhler made his illumination system public in 1893, about 50 years before the first anti reflective lens coatings.
Today the Köhler-illumination is still the ideal but you can start to do microscopy without reaching this standard.

[johntb]

Don't think one can achieve Köhler with very low magnification objectives - you should be ok on 10x and up though.

You're right. I should be starting with 10x as the lowest magnification objective.

Remember that you need to focus on a subject (using the brightfield port) as the first step of the process.

The empty turret slot is what is used for brightfield (i.e. no darkfield stop, phase contrast annulus, or Nomarski prism). Also, I should have mentioned that I'm focusing on a slide with a printed scale bar first.

Here is a tutorial that I find is both useful and fun.
Note: in the tutorial you need to begin by aligning the filament, something that you normally do just once (when you set up the illumination).

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

This is a good tutorial, but, after going through it, I'm still not sure why I'm not seeing the field iris diaphragm image with the 10x objective.

[johntb]

What type of condenser do you have and is it complete with top lens?

I'm using the Olumpus BH-2 UCD Universal Condenser. The specifications are given on page 13 of this Olympus BH-2 Brochure: http://alanwood.net/downloads/olympus-bh-2-brochure.pdf

It does have a top lens.

Have you tried it with a 40:1 objective?

No, I have a Nikon 40x objective, but the tube length is 210mm, while the Olympus microscope is built for 160mm, so I was concerned about that being another potential issue.

Also, I have a 20x objective (Olympus ULWD CDPlan 20, 0.40, 160/0-2) that I can try. The only concern is that the correction collar is stuck.

The condenser top lens should sit just below the slide.

I've been keeping the top swing lens out of the optical pathway. I suspect this may be the issue.

I'll try again with the top swing lens in place and see what happens.

[75RR]

I'll try again with the top swing lens in place and see what happens

I think you will find that that was the problem. The top lens should only be out of the way for low magnification objectives.

[Hobbyst46]

Along with the comments by 75RR and MicroBob above.

I tried to "simulate" your problem on my Zeiss scope:

I own an 1.3NA Abbe condenser as well as a 7-position 1.4NA achro.-aplan. phase contrast condenser that should be quite similar to your UCD.

I tried to blur or eliminate the view of the field iris diaphragm. I used an empty turret position or the "proper" brightfield position with the condenser iris fully open (they are entirely equivalent for this test).

The ONLY way that I DO NOT SEE clearly the field diaphragm opening, at its minimum diameter (I think about 2mm) when closed, is when the TOP lens of the condenser is "missing": either it is swung out (on the Abbe condenser) or screwed out and removed (for the 7-pos turret condenser).
The above holds for both the 6.3X and 10X objectives (I don't have an 4X).

[johntb]

It was the condenser top lens. It's supposed to be swung in the optical pathway for objective magnifications above 5x.

Thank you to everyone for the advice.