Bausch & Lomb Dynoptic long eyepiece identification

[hans]

I bought another Dynoptic with a straight vertical monocular body and this was included with it. The long lower section resembles the standard Huygens eyepieces with plano-convex lens at the bottom and field stop roughly in the middle. The larger-diameter middle section has another plano-convex lens focal length around 50 mm which focuses the field stop near infinity so the bottom two sections together seem to function like a ~5X Huygens eyepiece. The the upper section is just a hollow threaded coupler. All the threads are the same as the ones used on the normal eyepieces so for example an eye lens assembly from a normal eyepiece can be mounted in the top, but that does not seem to make much sense optically. Is this part of some sort of photo/projection eyepiece? Any ideas what would normally thread into the top of it?

[BramHuntingNematodes]

I have a B_L projective eyepiece if that era, with the original packing slip and everything. I can take a look at it tomorrow to see if it is the same.

[hans]

Yeah if you have anything resembling this I would be interested to see. I should check whether it is parfocal with the normal eyepieces but the Dynoptic is put away again for the moment, will try to experiment with it more this weekend.

[BramHuntingNematodes]

I believe what you have there is a projection eyepiece. The missing piece is a field stop with the magnification specified. Here are some photos of mine:

[hans]

That looks like the one, thanks Bram.

To make sense of the magnification... 5X I guessed in the first post is wrong because with the intermediate image between the field and eye lenses its size is modified by the field lens. I measured more carefully 55 mm field stop to eye lens. Then assuming standard Huygens design with focal length of the eye lens equal to (2/3)*f predicts magnification 250 mm / (55 mm / (2/3)) = 3.03 agreeing well with what is marked on the part missing from mine.