Converting Nikon UW trinocular for direct projection (1)

[Ichthyophthirius]

Converting Nikon UW trinocular head for direct projection: First impressions

Hi,

As there has been some interest in using Nikon trinocular heads for direct projection (placing the camera sensor in the intermediate image plane), I'd like to show you some (very early) results.

I bought a Nikon UW ultrawide trinocular head (unmarked as such) for use with CFUWN 10x/26.5 eyepieces a while back and now wanted to use it with a Nikon Optiphot 2. There appear to be many different versions of the UW head http://www.prc68.com/I/Labophot.html , this is one of them. This conversion might not be possible with other models.

This trinocular head has a very long photoport, so I assumed that by unscrewing the top (Nikon "chimney"), I would be able to place a Canon EOS 500D (APS-C sensor) in the intermediate image plane. http://www.photomacrography.net/forum/v ... hp?t=33227 However, no intermediate image was formed at all when the chimney was removed.

It turned out that there is a negative lens at the base of the trinocular head that projects the image at infinity! When a camera, objective focussed to infinity, is placed over the photoport, the image is formed again on the sensor.

At the base of the chimney there is a positive lens with a focal length of approx. 105 mm and an open diameter of 23 mm. The chimney is in effect a Keplerian telescope, that receives the image from infinity and forms an intermediate image again for the Nikon CF PL projective to pick up.

So in order to project the intermediate image directly onto the APS-C sensor, I had to place a positive lens (focal length between 100-120 mm for a decent coverage) inside the photoport. I measured the diameters of the light cones when using FN 26.5 eyepieces: 15 mm just below the negative lens; 35 mm at the top of the photoport when the chimney is removed. An acquaintance of mine did some calculations and concluded that the minimum open lens diameter for the achromat (when placed at the position of the original Nikon lens) would be 22 mm to avoid vignetting when trying to capture the entire FN 26.5 field of view.

As I couldn't place my homemage adapter quite that far into the photoport I chose a larger lens diameter; an achromat F = +100 mm, open diameter when mounted approx. 30 mm.

The Nikon chimney has an M50 x 0.75 external thread. I used a 50-49 adapter ring to anchor my homemade adapter to the inside of the photoport. The adapter is made entirely from pre-fabricated adapter rings; no custom-made parts needed apart from some cardboard rings, made with a circlecutter, to hold the lens in place. The achromat is sandwiched between two 32-42 mm step-up rings and a T2male-T2male intermediate ring. The upper section contains multiple T2 rings stacked on top of each other by using two T2 variable extensions: https://www.telescopehouse.com/revelati ... ff268.html They also allows for adjusting the distance between lens and EOS adapter.


Schematic of Nikon trinocular head; modified to illustrate the lightpath in the UW head


Unmarked UW head


Nikon "chimney" with lens at base


My homemade adapter


With Canon EOS mounted on photoport

[Ichthyophthirius]

Now for the results. The adapter works in principle, which is a good start. My Optiphot 2 has a 1.25x intermediate tube for DIC which I left in place for this test. The 1.25x tube factor narrows the visible field of the microscope; this is one of the reasons that I wanted to use direct projection, to get as much of the remaining field of the sensor as possible (FN 26.5/1.25 = FN 21.1).

The objective CFN Plan 10/0.30 gives very poor results. The edges are poorly illuminated; I think the NA 1.4 achromatic condenser of the microscope struggles to fully illuminate the Plan 10. My illumination might also not be properly set up yet. The results for the Plan 20/0.50 and the higher magnifications are much better.

The achromatic lens of the adapter introduces only very little field curvature as can be seen on the first photo. It shows an unstained mounted smear of human red blood cells in DIC. The image is in focus across the field, suffering only a bit in the corners. The Nikon DIC was not completely homogenous across this very large field (FN 21), that was not due to my adapter. I used a Nikon CFN Plan 40/0.70 DIC lens.



The second photo shows a stage micrometer in brightfield. There is some colour fringing here. On the whole, my impression is also that it is not quite as sharp as it could be (looks a bit "soft"). I used a Nikon CFN Plan Apo 40/0.95 Corr DIC lens at its sharpest setting. If someone here has an image of a stage micrometer taken with the Plan Apo 40 and a CF PL 2.5x projective for comparison, that would be really interesting!



From the micrometer reading (465 um), assuming that the nominal magnifications of the objective and tube lens are correct, 40:1 and 1.25x, the relay factor of my adapter is 0.96x.

The quality of the adapter, as well as the quality of the DIC, fall off in the corners, so using a lens F = 100 mm is probably a bit too ambitious. A focal length between 105 and 125 mm is probably more reasonable for anyone who wants to try this out.

[Ichthyophthirius]

Cons:
- image appears too soft
- possible introduction of colour fringes
- according to my acquaintance (who did the calculations for me), its hard to predict what lens design would give the best image here; some achromats might even give worse images than simple plano-convex lenses; more tests would be needed

Pros:
- simple and cheap adaptation
- fully reversible
- lower position of the camera makes it less susceptible to external vibration
- there is a chance that an open lens diameter of 22 mm is sufficient for this kind of adapter; that would allow simple 1.25" 0.5x "focal reducers" for telescopes to be used which usually have focal lengths between 97-107 mm

Things to do:
- try out more highly-corrected lens systems in place of the achromat; something like a 100 mm Tessar-type or the Raynox DCR-250 (125 mm focal length) might work; the problem is that it has to fit inside the photoport (maximum outer diameter: 49 mm); any ideas?
- reduce stray light: some flocking inside the adapter, a field stop, and something has to be done about the inside of the Nikon UW head: it is bare, unpainted aluminium (seriously!)

Regards, Ichty

[Sumguy01]

Thanks for sharing this info.
I would be interested to see more as you continue to progress.

[Ichthyophthirius]

Hi,

Thanks for your interest! I have maken a few more test images.




Filamentous alga with epibionts. Plan Apo 60/1.40 DIC, scale bars: 100 um and 50 um




Colonial diatoms. Plan Apo 60/1.40 DIC, scale bar: 50 um




Two choanoflagellates, very small epizoans. Plan Apo 60/1.40 DIC, scale bar: 20 um

.

[WaterGoblin]

Hi, I have a DIC nikon optiphot, I didn't know there was a DIC 60x objective. I'd like to know if it's compatible with my equipement, what condenser do you have?