BH-2 and darkfield

[Florian]

Hello everybody,

I have just acquired an Olympus BHS. I have the 4 “usual” Dplan and BH2-CD 1.25 Abbe condenser. I am still waiting for a BH2-PC condenser, a A 10X PL objective and a Splan apo 10X. I also have a vertical illuminator that I would like to test with neo objectives but they are too expensive for now.

First point:
When used in brightfield mode, are the phase condenser optics any different from the BH2-CD. I have been through a documentation stating that this condenser is using Abbe optics and I am wondering if I will see any difference ?

Second point:
I would like to add darkfield capability since it’s my favorite mode of contrast enhancement. I would like to obtain good result in both direct observation and photography. Right now, I am using my own finger, witch is not the best ^^.

I have learned about the following possibility :

1-The original BH2 or BX dry and oil darkfield condenser (BH2-DCD, BH2-DCW, U-DCD, U-DCW), they seems difficult to find and very expensive.

2-The BH2-PCD condenser with one darkfield stop giving darkfield capability all the way to the x40 (or 20x I am not sure). I know it depend of the objective NA and stopping down capability.An apochronaut post give a maximum theoretical NA of 0.65 for dry darkfield.

3-The amscope DK-200 condenser, either dry or oiled condenser, directly adaptable to the BHS.

4-Any random darkfield condenser than can be more or less easily adapted to the BHS condenser holder.

5-Adding a darkfield stop to the BH2-CD or PC condenser using a removable insert (or, why not, a permanent stop)
The two next link are example of this type of insert
https://www.mccrone.com/mm/machining-a- ... condenser/
https://www.alanwood.net/downloads/olym ... inberg.pdf

My questions are:

1-Can you achieve a good darkfield with oil condenser and low power/low NA objectives ? Correct me if wrong but I guess you loose a some light doing that . Is there any others considerations that make it impractical ? Is the BHS light powerful enough.

2-Is there any difference between using an insert with removable stop and a proper dry darkfield condenser ?

3-In this video you can see the stop used to provide darkfield in the BH2-PCD: https://www.youtube.com/watch?v=QSjlgNy ... lHunsinger.
Why people that use stop in a filter tray (like I was doing with my last microscope) tend to use different stop size matching objectives NA while the manufacturer go for one big stop for all low power objectives.

4-I was also thinking about switching one of the phase insert for a homemade darkfield stop. I still haven't try the phase contrast and I am not sure how useful I will find it. Do you know if this insert is made of a special glass in the original BH2-PCD. That don’t seem the best solution to me but why not.

5-What is, according to you, the best solution for darkfield with this microscope ?

6-What about the best solution for Rheinberg or oblique, Can I expect better result by permanently modifying a BH2-CD than with the Alan Wood insert Kit ?

Sorry for this long post and my uncertain English.
Flo

[Phill Brown]

Low power condensers are the least suited to DF.
Dry will be limited to 0.85 realistically but 0.75 is a more likely limit for 1.25.
Oil would be for 0.5 to 0.9,lower na objectives will only have the centre illuminated.
There are exceptions as always.
If you have a phase telescope and a slide ringing table you can make optimised patches.
Watch glasses with rings made using a fine tip marker will allow you to measure the diameter of the patch needed.
For better contrast and resolution the outer mask should be reduced to leave a gap of around 1mm.
Patches are usually placed near the iris.
There are already threads here to search for DIY DF.

[Florian]

Oil would be for 0.5 to 0.9,lower na objectives will only have the centre illuminated.

Thank you that help me !

For the DIY patch I have done some yet with no major problem. But I have never tested a dedicated dry darkfield condenser and was wondering if there is a difference in term of quality.