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  1. #1
    cooltouch's Avatar
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    De-cementing a lens element?

    I have an older Tamron Adaptall-2 500mm f/8 mirror lens. The secondary mirror's front element is cemented onto the front surface of the lens's primary element. I can see when looking down into the lens that the secondary mirror has begun to separate from the primary element -- sort of a rainbow-looking effect. The result of this is very muddy and soft images. The Tamron mirror is actually one of the better mirror lenses out there, and I know it can perform way better than it is. So I would like to de-cement the secondary and then recement it. I have tooling that I can use to align the secondary precisely for re-cementing, within +/- 0.001", which I think will be plenty good enough for this.

    But what I need to know is, what is a good substance to use that will most likely soften the existing cement so I can remove the secondary without damaging anything, and what sort of new cement do you think would be best to use?

    The secondary is surrounded by a plastic cover that cannot be removed. At least one decementing agent I've read about requires total immersion of the element(s) and heating them to 340F. Something tells me that the plastic cover will not survive this

    I'm hoping there might be other methods where maybe I can, say, wick it inside a (hopeful) space between the elements with a toothpick or q-tip or something?

    TIA
    Last edited by cooltouch; 12-10-2011 at 05:27 PM. Click to view previous post history.

  2. #2
    keithwms's Avatar
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    Hmm maybe acetone. You might also try baking the pair, that might separate them due to the difference in thermal properties. Don't blame me if something breaks though
    "Only dead fish follow the stream"

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  3. #3
    cooltouch's Avatar
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    Or melts.

  4. #4
    John Austin's Avatar
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    Look on eBay - Messing around with lens cement removal is too much effort

    However, xylene should work, but unless you are getting too old to worry about kidney cancer (printmaker's cancer) I don't recommend it - I trained as a fine print etcher, but returned to silver jelly photography very quickly - I know a lot of etchers and screen printers who are now dying from kidney or liver cancer - So use your current lens as a paperweight and get a good SH one

    John

  5. #5
    cooltouch's Avatar
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    Actually I don't quit easily. If I were to do this, I would do it outdoors. I even have a respirator I use when spray painting my motorcycle tanks, etc. And I have rubber gloves for substances that don't see skin as much of a barrier to getting into the bloodstream. I just don't like the idea of having to bake things because the plastic cover surrounding the secondary element wouldn't survive.

    I'm sure Tamron never intended for this sort of lens to be repaired. If it would have been a warranty situation they would have just replaced the assembly and thrown away the defective one. But that doesn't stop me from wondering if its possible.

    I don't really have any use for the lens. I bought it from a municipal surplus auction along with a couple of other lenses, but I bid on the lot because of the Tamron. Still, I'm not in it for much. I have since bought another Tamron 500mm mirror, so I don't really need it anymore. So, I was thinking I'd just sell it on feebay "as is," fully disclosing its problems, but it also occurred to me that, if I could repair it, why not? I could then sell it for a bit more than I would otherwise get for it. That's all, really. And, hey, in the process, doing the repair itself would be kind of fun, I'm thinking.

  6. #6
    John Austin's Avatar
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    Michael - Have fun, but please take care and check the plastic bits are not soluble in your selected solvent - If the whole thing ends up as useless bits at least you will have something to laught to yourself about

    PS Please send me a message to let me know how you get on - I am much better a giving good advice than following it

    John

  7. #7
    Ian Grant's Avatar
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    Have a search on the Large Format forum, there's some excellent first hand advice on this subject.

    I have alens, early 150mm Sironar with separation and if I ever get it back from the Italian APUG member I lent it to I'll follow the advice Stven Tribe and others gave,

    Ian

  8. #8

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    try Norland Optical Adhesive 61

    NOA 61

    You could wick-in some but it is UV set.
    You could use direct sun light but I don't know how you would deal with run-out.
    Yes, methylene chloride would melt your plastic piece.

    Good luck with it.

  9. #9

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    Before you go through the rigamarole of that whole process, try simply filling the separation with oil. Get a very fine oil and put a drop on the outside of the separation and leave it wick in overnight. Often this will fix the problem and you won't even be able to see where the lens was separated. The refractive index is about the same too so you don't lose any optical quality. I have done this several times before and it really works. I have also re-cemented lenses before too and that is a real pain to do. Give the oil a try. You have nothing to lose except a few minutes.

  10. #10
    cooltouch's Avatar
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    Thanks for the responses, guys. M Lointain, yours sounds like a great idea. I have some very low viscosity machine oil -- it's almost like water, it's so thin -- that might just be perfect. It is just the slightest yellow color so it shouldn't affect color rendition. I also have some insulin syringes around here somewhere that I use to inject this oil into tight spaces. They're great for tight work because the needles are hair thin. I don't recall where I put them now. I was looking for them the other day and wasn't able to find them. Guess I need to track them down now.

    So I'll give this a try and I'll report back to y'all on the outcome.

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