Repolishing old lenses with fungus, balsam separation

[kfed1984]

Damn that was a good post! I will read again several times. MEK is Methyl Ethyl Ketone? If so, I just happen to have one gallon of the stuff for a different purpose, that didn't materialize. Do you have anything against real balsam? Auto glass repair resin? Never heard of it. I now like the balsam because it keeps well. Also I considered 3D printing clear uv resin. But I think its a thermoset polymer what cannot be reheated and melted again.

[Tin Can]

but what about the FUNGUS?

[Mark J]

Yes, great info from Kevin.
However, here's a question, going back many steps, has anyone just tried oil ( preferably microscope immersion oil ) applied to the gap where the cement has separated, to see if it will wick into the gap and cure the problem optically ?

[Kevin Crisp]

I haven't tried oil, though I suppose it might help in some situations. But, again, if the issue is some relatively minor separation on the edge, the lens is probably going to work well anyway without it.

I never considered or looked into using balsam, figuring there's a modern product for this that is stable and long-lasting so I'll use that. I like the fact that I can check my work with the UV-cure cements before hitting them with a fully curing dose of light. So I just don't know how to work with it. I have nothing against it.

Lenses can have a lot wrong with them and still work acceptably. I bought a compromised 360mm Wollensak protar-copy. (Coated, in my experience the best Protar version anybody ever made. I think they called them Series Ia) The front element didn't look right. I took it out of the mount and found that one of the formerly UV-glued surfaces had come apart, been cleaned off, then the lens reassembled. Whoever did this wanted the now-separated glass surfaces to be as close together as possible in the mount, but not quite touching. So at 90 degree intervals he/she put little pieces of mylar tape on the edge as extremely thin spacers. I tried it out before I fixed it, and at f:16 or lower it wasn't bad at all.

[Steven Tribe]

This method of tiny spacers at the edge was used by early telescope objective makers. I have discovered them when renovating some about 10 years ago. They were silver coloured metal - tin, I think. I have also seen oil used as an optical contact in early telescopes.

[Mark J]

I haven't tried oil, though I suppose it might help in some situations. But, again, if the issue is some relatively minor separation on the edge, the lens is probably going to work well anyway without it.

Well, not necessarily, it depends on the lens design. The failure mode would be that for high field angles, the light doesn't get out of Lens 1 into lens 2 , it just TIR's ( Total Internal reflect ) on the rear of lens 1 ( the positive).

[Kevin Crisp]

Well, "I suppose it might help in some situations" and "the lens is probably going to work well anyway" do not, in my opinion, suggest that it would "necessarily" have no impact. I was assuming relatively minor separation and use at smaller apertures.

[Mark J]

I'm not trying to get at you, Kevin. I'm just coming at this from a different angle, being a designer.
Re. separation, I was thinking in the context of the OP's 300mm Symmar picture.

As it is, i went away and checked a 210 Apo-Symmar model, and the full field rays for f/5.6 or f/22 don't get that close to the edge of lens 2 , so it's quiet tolerant ( as would a Symmar-S be, most likely ) of a fair bit of edge separation. A typically tele would be less so.

[Bayliss]

I'm planning on repairing some balsam separation, but can't decide on the best adhesive. Is the Canada Balsam you can get from art supply sites (e.g. https://www.cornelissen.com/oils/sol...da-balsam.html) the same as the Canada Balsam sold for microscopy? I haven't been able to find a source for the latter (many sites appear to stock it but will only sell to businesses).

Also has anyone here tried Norland Optical Adhesive 60 (https://www.norlandprod.com/adhesives/noa%2060.html)? It was designed as a cement for lenses and looks pretty good, I'm hesitant to use a UV glue though because they seem so much harder than Canada Balsam to reverse. Thanks!

[Mark J]

Norland cements like 60 ( and 61 ) are widely used in industry, but maybe aren't the best choice for home use. Their expiry date is usually short and you need UV lamps.