Adjusting the condenser

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1

Hi there,
I just got my first OFM up and running and am deeply impressed, really brillant work!

However, I still have one issue: Regarding adjusting the condenser position, in the assembly documentation it it stated that i need “not worry about the exact position, this will be adjusted on first use”. But I could not find any information about how to do that. Did I miss anything?

Best,
Michael

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2

Glad to hear you have your microscope working. Which version is it?

The setup and use part of the instructions is a little sparse at present. Unless you are using a high-NA objective (60x or more) the illumination is not critical. Put the condenser about 5-8mm above the slide, where the light seems to be best focused. I find putting a piece of paper where the slide should be helps to see when it is focused. You then need to loosen the screws holding the illumination dovetail a little and move it so that the illumination is centred on the lens. You can get this about right by looking at whether the spot looks to be over the centre of the lens. For fine adjustment, first disable the flat field and colour correction (in settings) and then adjust to get the brightest part in the centre. This can be a little tricky, but it only needs doing once. When this is all done, re-do the full camera autocorrection.

For high resolution imaging the numerical aperture of the illumination needs to be at least as high as the NA of the lens. This requires more care in getting the condenser at the right height.

3

Thanks for your input. I just built the v7 beta OFM.

Adjusting the condenser with a high NA objective (100x/1.3) is exactly my problem. Is there a general approach to do this with the OFM, or should I just move the condensor position until the resolution looks fine? Currently, the light cone is not even really centred with respect to the objective lens, but maybe I could fiddle a little bit with the LED position. The illumination is the kit from taulab.eu.

Best,
Michael

4

You should be able to centre the illumination by adjusting the position of the illumination dovetail as above. That is actually slightly easier for high magnification as the field of view is quite a bit smaller than the illumination. @JohemianKnapsody might have some tips for getting the height right.

5

I would:

  1. Start up the microscope with no sample.
  2. Place a piece of white paper over where the sample goes.
  3. Adjust the height of the condenser until the spot is small and focussed.
  4. Remove the paper.
  5. Slightly loosen the M3 screws that hold the bottom of the dovetail to the microscope body.
  6. Try to shunt the illumination until the focussed spot is over the centre of the objective.
  7. Now look at the microscope image feed.
  8. Fine adjust the x-y position until it is centred.
  9. Tighten the M3 screws (this can cause the x-y position to shift a little. If it does you can loosen, readjust, and re-tighten)

The x-y adjustment is the worst part of the of the process. But it should only need doing once.

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6

Thanks for your advice, I managed to adjust the condensor position nicely, moving the dovetail did the trick :grinning:

I set the condenser to a height approx. 7 mm above the objective lens (100x/1.3), to have enough space for wiping the oil off the objective. I think thanks to the diffuser the angle at which the light can enter the objective is large enough to make use of the NA, as the optical resolution is close to what I would expect. However, the illumination spot still looks much to large for my feeling; with this setting, the diameter of the illuminated spot is 5-10 times larger then the diameter of the lens. I wonder if the contrast would become better by having a smaller light spot, but since there is no field diaphragm, I see no other way to reduce the amount of stray light than getting the condenser even closer to the objective.

Would you mind to put in your two cents about this, and do you have a recommendation on the optimal distance between condenser and objective?

To avoid smearing oil on the condenser lens (which at some point is bound to happen when cleaning the objective if the distance is too small), maybe it would be worthwhile to think about a simple solution to fold the condensor out of the way for cleaning. What do you think?

Best,
Michael

7

We never really got around to playing with apertures enough to put them in the instructions. But I designed the condenser to have a large aperture that can then be adjusted. As we are working in a tight space it didn’t seem possible to have a diaphragm aperture. So the plan was to have swappable apertures.

If you go to the customisations and alternatives page, there is a link to download every STL. Inside this there is a “condenser aperture”. This can be slotted into the condenser.

I wouldn’t trust the hole in the aperture to be the correct size when 3D printing. So the cone down to the hole is 118°, this is the same angle as the top of a drill bit, so it should help a drill find the centre. If you have a set of small drill bits I would print a few apertures and play around with the sizes. It isn’t something I really ever had time to finish to my satisfaction.

I would recommend doing the drilling on a scrap wooden surface with an Archimedes drill.

@WilliamW is there an issue in the GitLab about finishing the condenser aperture and adding it to the instructions?

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There is not an issue listed on the aperture currently. We have discussed it and it does not currently lie on the path to the v7.0.0 release, I think it will need some proper thought that we do not have time for now. You are right though, it should be noted in an issue so we don’t forget about it as it would be a very useful feature.

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I agree it doesn’t block v7 release. It is also a nice issue in that it doesn’t add major changes that affect operation, so if someone did work on it, it could be included before the v7 release.

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