Iris+condenser troubles (I need the help of the boards real brain thinkers on this one)

[BramHuntingNematodes]

So some prankster accepted a joke offer on this here B&L 1.4 NA achromat condenser what with the thick slide requirement. Research from the 1953 Accessories catalog suggests this is the research type. The only difference between it and the ring condenser version I believe is the black lacquered tube rather than the brass sleeve (oh that black lacquer does look smart). In any case, it advises that the same iris diaphragm be used for either, catalog no. 31-58-28. "Well," I think, "it's probably the same as the one off of this roughly contemporaneous Abbe with a crack in the top I was using before I mutilated that Wild M11 apl on my rickety lathe, so let's use that." The do have the same size thread:



However there is a slight difference in that the optics of the achromat poke out below where the iris usually is:



Grrrr! I can't screw it on!!!



The diameter is roughly 32 or 33 mm, does anyone know what the thread pitch and proper diameter of this is supposed to be? A 5mm extension tube would do the trick. What else am I missing?

[apochronaut]

I'm pretty sure Bausch & Lomb used inch sizing for most of their tooling but it is possible that they switched over to metric later on for some things. I ended up with quite a few odds and ends of B & L condenser parts from a job lot I purchased in order to get some special AO parts. I certainly have several of that 1.4 N.A. condenser and multiple iris diaphragm units.
I do recall from the past as well, with an old research microscope that had a euchred diaphragm, that I tried to swap the condenser over to another diaphragm and the threads were different. I will check either today or Sunday for you and see what I can find out.

B & L made several versions of that 1.4 N.A. achromat/aplanat, with the difference being in the focal length. That was the determining factor in the specification for slide thickness. The ones on an akehurst slide such as yours were used on the DDE, with several condensers being critically tuned to each apochromatic objective.

[BramHuntingNematodes]

Thanks Apo!I am sure the threads are the same as I took the flange off the iris and was able to thread it onto the condenser. If I got some longer screws and a big nylon washer that might give me enough room, but that seems like an inelegant solution.

I am hoping it works OK with my 20x and 10x-- I would prefer to make a dust hood before taking apart the 45x. I don't know if it's my eyes playing tricks but the lower collector lens on this condenser appears to have some kind of optical coating, which has not been the case with any of my other optics up to this point. The most dramatic improvements I have been able to squeeze out of this old scope have been from changes to the substage. On the other hand, any improvement over the Wild will likely be subtle as it is a very good piece of kit.

[apochronaut]

So, I found two solutions for you in that box of B & L condenser parts. The 8.9mm focal length 1.4 N.A. achromat you have, has a lower section slightly different than the version that would have been used in a sleeve mount, rather than in an akehurst slide, as yours was used. The neck of the sleeve/collar mount version has a thin, threaded black extension on the bottom, similar to the abbe, that gives enough clearance from the iris. There is an adapter ring that replaces it, if it isn't there.
There also was an alternate version of the iris section with a slightly thicker housing that keeps the lens section above the iris when threaded on. I have one of those too.

both or either are available , if you want.