How to disassemble AO/Reichert 1201 condenser?

[hans]

I have one with a bunch of oil in it. I took out the threaded retaining ring on the bottom and the lower lens could spin the lower lens freely with my fingers but it was not threading out. I then tried pressing on the top lens to break the adhesive between the conical surface and housing. Probably should have asked first because I think I fractured the top lens internally while pressing on it. In any case I would still like to get it apart out of curiousity. Has anyone successfully disassembled one of these?

[hans]

Some photos in case anyone unfamiliar with the 1201 has suggestions. I have checked the housing very carefully with a stereo microscope and cannot see any joints, pretty sure it is a single, machined piece. The narrow internal ledge visible in the bottom just past where the threads end also appears to be a continuous part of the housing, not another retaining ring.

[apochronaut]

You should be able to see a bit of substance along the edge of the bottom lens once the retaining ring has been removed. My guess is that there is an oil seal there. The lens is possibly being held in by vaccum. The doublet should just drop out once the retaining ring has been unthreaded. The front lens is either press fit or cemented, so will stay in place and has no physical contact with the bottom doublet.
Try some mild solvent : maybe something akin to varsol, will thin the oil enough to allow the weight of the lens to overcome the resistance. It is quite thick and heavy.
I don' t see where it is fractured?

[hans]

Thanks, had not considered vacuum due to all the oil, will try solvent, maybe I can also try grabbing the bottom lens with suction and pulling. The fractures are not very obvious in the photos, more obvious in person, I circled them in red:

[BrianBurnes]

I've often seen camera lens repair folks use suction cup tools to remove stubborn lenses held in by friction or vacuum. They are gentle on the optics but can apply a lot of force. There are tools made for the purpose such as this one, but even household suction cups from the hardware store (e.g. for mounting things onto shower tiles) can do quite well.

[apochronaut]

Yes, I see them now. They may be outside the optical path required for .90. It might still work o.k. It's not the first one I have known to have been drowned in oil, although it is the first I have seen where it invaded.

[hans]

You should be able to see a bit of substance along the edge of the bottom lens once the retaining ring has been removed.

There is what looks to me like a joint with another element of slightly larger diameter and fillet of adhesive which I have circled in red. That all rotates together when I spin it. Do you know if the bottom lens is a doublet?

I've often seen camera lens repair folks use suction cup tools to remove stubborn lenses held in by friction or vacuum. They are gentle on the optics but can apply a lot of force. There are tools made for the purpose such as this one, but even household suction cups from the hardware store (e.g. for mounting things onto shower tiles) can do quite well.

That tool looks useful. Just checked, the standard suction cups I have are too shallow to conform to the rather extreme curvature of the bottom element. I do have one of those little hand vacuum pumps for automotive troubleshooting, will try to find something I can use to suck onto the lens.

They may be outside the optical path required for .90. It might still work o.k.

Yeah, hopefully, hard to tell with all the oil in there.

[apochronaut]

I meant that the fractures may be outside the optical pathway, once you get the oil out. I can't see it working well with oil hanging around in there.

Yes, the bottom lens is a doublet. The seam you are seeing is because the bottom plano convex element is slightly smaller than the top plano convex element, although I really don't know if the mating surfaces are plano. I assume they are.

[hans]

Not much luck with suction so far. I can probably apply a little more force if I find something with diameter better matched to the lens. The lens definitely moved some but now appears to have wedged against something on the way out and no longer spins freely.

[apochronaut]

Cocked slightly, I guess. For a lot of cocked lenses or just tight ones, I use a firm downward crack on a hardwood block or cutting board. If the lens is just firmly seated, I hit the board exit side down as level as I can. If the lens is cocked I determine the low side and try to hit the board at a slight angle, heavy on the high side. Often it works with a few trys but if you reverse the high and low, it can make it worse. I put a tissue on the board but not a cloth.

[hans]

Yeah, probably right about the lens getting crooked, as evidenced by the chip circled in red. Your technique worked nicely. The bottom lens was protruding slightly below the housing so couldn't be hit directly. I taped the condenser in between two sockets which allowed knocking from either direction. Some tapping from several angles on the top to reseat the lens, then a single hard smack on the bottom dropped it out. Now that I have it apart it looks like the top lens inserts from the top into a cylindrical recess. No way it could insert from the bottom with a conical mating to the housing as I was thinking, as the diameter of the opening in the housing below the top lens is smaller than the opening in the top of the housing. I am thinking it looks like in this crude cross-section drawing, where red is adhesive, so the pressing from the top was dumb idea. The internal interface of the bottom element does appear to be curved, as in the photo with some oil on the ground edge to get a better view, and the violet laser gives that impression also.

1201-abuse.jpg (99.93 KiB) Viewed 4362 times

1201-disassembled.jpg (108.51 KiB) Viewed 4362 times

1201-top-section.jpg (97.66 KiB) Viewed 4362 times

1201-bottom-doublet.jpg (116.46 KiB) Viewed 4362 times

[apochronaut]

On reflection the fact that the lower lens has a smaller diameter than the upper would indicate that the upper lens is a fairly severe negative meniscus.

[hans]

I reassembled it after cleaning out all the oil and don't see any obvious problems compared to my other 1201. Both the field and condenser (viewing with phase telescope) diaphragms focus sharply with good contrast with the 4X and 100X objectives. Looking through the condenser alone the fractures do appear within the field of view a bit, perhaps what saves it is that in normal operation the full illuminated field diameter and NA capabilities of the condenser are not used simultaneously? (5 mm at NA 0.1, 0.2 mm at NA 0.9, and other combinations in between...)

[apochronaut]

The lens diameter would control the field coverage/N.A. ratio . That condenser was originally introduced as a phase contrast condenser in the 34mm parfocal system, so had limited use at low N.A./ magnification and wide field, still it will fill a 20mm f.o.v. at 4X N.A. .12, which appears to be near it's limit. It won't fill a 22mm field. It was obviously designed so that the phase carousel could be completely utilized by adding a 4X .12 objective to the nosepiece and not have to resort to using an aux. swing in condenser, as was necessary with the 1087 abbe aspheric.
Later the 1201 was also utilized as a phase condenser for the 45mm parfocal system as well as a general purpose oil free condenser for the same system.

The # 1242 phase condenser was used as the phase condenser for the older 34mm infinity parfocal system ; so in fact with the same objectives as the 1201 was catalogued for in it's 34mm era , although there were some alterations to some of those objectives without changing the objective catalogue numbers, for the series 100 phase scopes. It has no description of type, just a catalogue #. At least I have never found one. Someone must know.
It is also a 3 element in two group condenser, looking very much like the 1201. The bottom element also has a double convex profile but the upper surface is very subtly convex and appears to have a slight aspheric curve. Very subtle, almost plane. The common surface is the reverse of the 1201 . It is the lower element that has a negative meniscus and the upper a double convex with the lower element being of a slightly wider diameter than the upper. The negative meniscus curvature probably has a thin flat ground surface.

[hans]

Any thoughts on whether the 1201 top lens could possible be a doublet also? Looking at it with the violet laser is a bit confusing. Fluorescence from the glass appears to stop part way through, consistent with a change in glass type, but I do not see any distinct fluorescence from the adhesive itself which I have seen in all other cases, including the bottom doublet of the 1201. The bright spot is the opposite surface which I fogged with my breath to make it more visible.

[apochronaut]

It's entirely possible but any schematics I have seen of a composite front condenser lens show a fairly shallow disc cemented to a relatively thicker plano/convex lens.