Anyone have experience disassembling a Voigtlander Septon 50mm f/2 DKL mount?

[dcshooter]

Hey everyone,

I've managed to acquire a sample of this lens which is in great condition except for one problem: major delamination in both cemented doublets.

Here's a lens diagram for reference:

I've managed to extract the rear doublet and separate/recement it, but the front one is giving me a huge amount of trouble.

I've managed to remove the front two elements and am confronted with this:

That sure as heck looks like a small black retaining ring on the front, but I've had zero success removing it with heat/solvents/various rubber tools, etc. Unlike every other retaining ring in the lens, it doesn't have spanner notches.

Looking at it from behind, it looks like this. No obvious retaining rings on the back either. It looks to be all one machined brass piece.

I've managed to find the following page where a Chinese repairer seems to have managed to take this group out:
http://lens-cla.blogspot.com/2014/04/voigtlander-septon-50mm-f20-lens-group.html

Here's his picture of the removed doublet:

I've tried contacting this guy (in English) with no success, and running the page through Google translate seems to reveal only a general description of disassembling the lens.

So, has anyone here managed to similarly disassemble this lens? If so, do you have any tips/tricks as to how to get this dang thing apart?

Thanks!

 

[02Pilot]

Before you go any further, have you tried shooting it to see just how much effect the separation has on photos? I've got a lens with major separation right in the center, and it's undetectable in photos.

Beyond that, the groups look like they might be swaged in - if so, you'll need a lathe to get them apart.

 

[dcshooter]

It's bad.
The separation covers nearly the entire element and drastically reduces the contrast of the lens.
Swaging seems like an unnecessarily dangerous operation so close to a glass element, but I suppose it's possible.

 

[02Pilot]

That's unfortunate.

The only possible split I can see is directly above the threads in the second photo - there's a thin piece of bare brass that might indicate a split.

Other than that, have you considered (slowly and carefully) heating the assembly to see if you can reflow the balsam without separating it? Probably a last resort, but if you get to that point, there's not much to lose.

 

[dcshooter]

Unfortunatly, it's an epoxy-based cement, and quite a tenacious one at that. It took a couple of weeks in methylene chloride to separate the other group. Here's an example of what it does to images:

If you If you look below the hummingbird feeder, you can clearly see the better contrast in areas where there are still traces of cement adhered vs where it has become air-gapped.

I'm not a big fan of the approach, but what I might do is try to flow some oil in from the sides using capillary action to see if I can at least get rid of the air gapping. Not a permanent solution of course, but I bought the lens to use, not as a collector's piece, so if it works, it works.

That's unfortunate.

The only possible split I can see is directly above the threads in the second photo - there's a thin piece of bare brass that might indicate a split.

Other than that, have you considered (slowly and carefully) heating the assembly to see if you can reflow the balsam without separating it? Probably a last resort, but if you get to that point, there's not much to lose.

 

[02Pilot]

Ugh. I so prefer balsam over other cements in this application.

The oil may be your best bet. What sort of oil are you thinking of using? I'd think a synthetic might be your best bet, probably something moderately viscous. Warm it up to make it flow better (100C is probably a good temp - warm the lens too to minimize thermal shock).

 

[dcshooter]

I'm thinking a soak in methylene chloride for a couple of days to see if I can get the last bits of cement to release, then a couple of days in acetone to remove any gunk trapped between the elements. As far as oil goes, I've heard of everything from wd40 to salad oil(!) being used. I get the sense that a thinner, more penetrating oil might be the best bet in terms of getting a uniform join with no air bubbles, but I suppose it will tend to be less permanent. I have some Norland Uv cure optical adhesive that I am tempted to try to put on and then seal the assembly in a dark room for a few days to see if i can get it to naturally flow between before uv curing it. It definitely seems to have good capillary action on clean glass. If it works, it could be a permanent solution. The problem is that if it penetrates only partially, it may be weeks back in the methylene chloride to get it back out.

 

[02Pilot]

Yeah, no matter what you've got some significant risk of failure. I'd go for the oil first, simply to see if it's a viable option to get the lens into action. If it works, great; if not, it's a lot easier to clean up than the optical cement.

As far as the oil, I would avoid WD-40 - it's mostly solvent. Salad oil might work, though I can't imagine any advantage to using it. I'd probably try either a synthetic firearms lubricant or sunflower oil - both flow very well, with the synthetic probably being the more stable of the two. Baby oil is very clear and might serve your purpose as well, though the lens would inevitably smell funny.