Olympus U-DCW darkfield condender parameters?

[TomF]

Hi,

does anybody please have any information regarding construction and parameters of Olympus darkfield condenser U-DCW? I bought one last year for our laboratory and I´ve been intensively trying to implement it in my microscope recently. I am frustrated that I can´t find just any information about the condenser except it is of cardioid construction and numerical aperture is 1.2-1.4. I somehow measuered the dovetail dimensions and made an adapter for threads we use via resin 3D printer. I would especially appreciate following parameters:
- ideal slide thickness (I´ve been using 1 mm thick slides but it seems somehow thicker than what the condenser is designed for)
- ideal beam diameter (I estimated roughly 22 mm of the ring aperture inside the condenser)
- focal length would be nice

Overall, any personal experiences with the abovementioned condenser or other high NA immersion condensers are much appreciated.

[Hobbyst46]

While I do not know this specific condenser, I feel that 1.0-1.1 mm slide thickness should be OK. But, on some microscopes, the vertical movement of the condenser is limited, by a blocking screw, such that it cannot be sufficiently raised to nearly contact the slide. For DF, the condenser's top lens must nearly touch the slide. A drop of immersion oil on the lens then forms the correct hollow cone beam.

[TomF]

I've been using the condenser in a microscope prototype which I built on optical table so I am not limited by any screw in vertical position. Yet, even in case of the absolute contact of the condenser with the oiled slide, it seems to me that the image of the source made by the condenser on a frosted part of the slide is slightly out of focus so I am wondering whether or not is the slide thin enough? Thinking about it, I can try to check for this briefly if I defocus the illuminating beam to a slightly diverging beam so the image of the source would somehow shift further behind the condenser. Still, this is just a half-way fix, because I suppose that it would introduce aberrations to the illumination path. I am already able to get quite nice dark field but the reason I pursue perfection in the illumination beam is that I´ve been observing defocused golden nanorods so I need 1) absolutely the best S/N I can get 2) all of the energy I can get exactly to the sample plane.

[PeteM]

Just to be sure of a couple of things. I'm guessing you've checked all these, but just in case . . . Are you oiling the condenser to the bottom of the slide as well as the cover slip to the objective? Are you using something like a 100x oil iris objective, so you can reduce the numerical aperture to below 1.20 if need be (darker contrast)? Is the illumination path of your rig set for near perfect Kohler illumination at the specimen plane?

[TomF]

I am using oil condenser with dry objective (Olympus UPlanApo 60/0.9) so I am oiling only the condenser. I am aware (or rather I expect) that I would get more light with proper objective with maximum aperture of something like 1.2 but the only objective we have with higher aperture than I´m using right now is Olympus 100/1.4 with no additional iris so it is a no-go for me and my sample is a thin layer of water with nanorods with coverglass freely sliding on the layer so oiling potential immersion objective to the coverglass would be somewhat cumbersome to use and refocus.

BUT - PROBLEM FOUND
Today I spent a few hours with my frankescope and found two things:
1) I am using some overpriced Thorlabs metal nonsense as microscope slide holder. On the side of the slide, where the microscope slide is frosted, which I´ve been using to fine tune the centering and vertical position of the condenser, the holder is not as wide as in the center and if the condenser top is not precisely in the midle, the metal border prevents the condenser to get close enough to the slide. This is just minor issue added to the major one, overcame by paying attention during adjustment of the condenser position.

2) The problem is that I´m reimaging the source plane to the iris which I planned to use as a field iris for the illumination. Next, the iris is collimated with a doublet. The image of the source created by condenser, which seemed defocused at first or with a halo I found is a primary image of the source, as desired, but with a defocused secondary image of the source. The reason is probably undesired reflection caused by the collimating doublet. Solution would be to use at least a singlet, or better parabolic mirror in place of optical element colimating the field iris, avoiding all potential parasitic reflections. Unfortunatelly, I currently do not have either of these. A colleague of mine promised to try to get one for me by monday. By then, the frankenscope will hopefully by finaly fully operating on its limits.