Polishing lens glass

Discussion in 'Plate Cameras and Accessories' started by illumiquest, Jul 23, 2013.

  1. illumiquest

    illumiquest Member

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    I have and ancient CC Harrison full plate lens which I've been using for two years. It does a beautiful job when I use strobe but does have a ton of cleaning marks on it so outdoors it's very soft and hazy. I'm wondering if there's a good way of cleaning up these marks and making it more useable for natural light?
     
  2. E. von Hoegh

    E. von Hoegh Member

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    This isn't a DIY job unless you have a lot of skill and at least some experience. You would have to preserve the original radius of curvature while maintaining centration of the surface, to a tolerance of about nothing.
     
  3. Gerald C Koch

    Gerald C Koch Member

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    In my experience cleaning marks tend to be rather deep. Probably too deep to be fixed without professional help. The front element would probably have to be reground or replaced. You can try very carefully filling in each deep scratch with a bit of black paint. This will prevent the flare problem outdoors. The lens will look like hell but work better.
     
  4. E. von Hoegh

    E. von Hoegh Member

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    Also, I forgot to mention that a good lens shade of the compendium type will make a great, perhaps huge, difference. :smile:
     
  5. vysk

    vysk Member

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    If you are daring, make good friends with a hobbyist telescope maker and give it a go.

    It ain't rocket science, just something that takes care and attention to detail.
     
  6. pen s

    pen s Member

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  7. E. von Hoegh

    E. von Hoegh Member

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  8. Ian Grant

    Ian Grant Subscriber

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    Cerium oxide isn't expensive, I bought some from a Lapidiary suppliers earlier this year and they told me they only sold it these days to amateur telescope manufacturers and similar. I wanted to try some on a Ground glass screen and found that it polished the glass rather quickly.

    Ian
     
  9. Tom1956

    Tom1956 Inactive

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    Now, if only somebody has come up with home-made lens coating, we've got it made.
     
  10. lxdude

    lxdude Member

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    If it's an ancient lens, it had none originally. And single coatings are available. I can't remember now the Leica guy who could repolish the older Leica lenses and redo the coatings as part of his repairs/restorations. He would recoat any lens, as I recall.
     
  11. Tom1956

    Tom1956 Inactive

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    Oh, yeah? I'd sure like to hear more about this. Certainly there's a way to do this without a jillion dollars worth of equipment.
     
  12. E. von Hoegh

    E. von Hoegh Member

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    Piece of cake. You'll need a vacuum pump, a bell jar and plate, magnesium flouride and a way to vaporise it in vacuo. The same equipment is used to aluminise telescope mirrors.
     
  13. Tom1956

    Tom1956 Inactive

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    So, MgF2 has a boiling point of 4100 F. That's pretty hot. So you'd need to achieve this temperature and get the gas inside a container with your lens, like cigarette smoke on your windows. Hmm... I need to think this thing through. A bell jar seems easy enough to come up with. A vacuum pump could be the intake side of any air compressor. I saw a fancy machine on ebay for several thousand dollars, but certainly don't have a pile of cash I can peel off a few bills from. Certainly this can be done at home.
     
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  15. E. von Hoegh

    E. von Hoegh Member

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    Wrong. You need a vacuum, not a region of low pressure. A compressor won't even come close. You might be able to modify one though.
    As I said, you need a way to vaporise the stuff in vacuo. That means inside the bell jar under vacuum, so it can condense in the surfaces you want to coat. Think of electricity.
     
    Last edited by a moderator: Jul 26, 2013
  16. Tom1956

    Tom1956 Inactive

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    So if this process is akin to silvering telescope mirrors, shouldn't I be studying the home-making of telescope mirrors? All I'm saying is certainly this can be done without going into the poorhouse. Thanks.
    BTW--I've got my customer shipping me a quart of methylene chloride on the lens separation project. Acetone and boiling were worthless. My buddy want to put it in his pressure-cooker. I'm a bit more conservative and patient.
     
  17. Tom1956

    Tom1956 Inactive

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    Sorry OP for hogging your thread. I wonder if this vacuum would have to be as high as the inside of a radio tube.
     
  18. Tom1956

    Tom1956 Inactive

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    Lets see... An old Nuarc platemaker uses HV to strike an arc with carbon sticks. This causes smoke. The smoke fogs up the vacuum frame glass. Substituting MgF2 in this analogy, you would want the gas from that to deposit on your lens. From that point the coated lens is to be baked to get the deposit like glazing pottery.
     
  19. NormanV

    NormanV Member

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  20. E. von Hoegh

    E. von Hoegh Member

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    Carbon arcs are typically 35v - 90v, and use high current not high voltage. The vacuum would be a few mm of Hg, not as high as a radio tube. The vapor deposits on pretty much anything cooler than it, you have to shield what you don't want coated. Electron beams have been used to heat the coating materials, remember Fuji EBC coatings?

    The trouble is that anything in a vapor phase will deposit on anything cooler than it, so I don't think a carbon arc would produce a clean enough MgFl vapor to result in a useful coating. It would sure supply the heat though.
    Look into the work done by Katherine Blodgett for Generous Electric.
     
  21. Tom1956

    Tom1956 Inactive

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    I am seeing nothing of value on the net to help me figure out any kind of feasible DIY coating. Certainly there must be a way. Seems to me simply getting a layer to deposit would be half the battle. Seems like there would have to be a way to "set" the coating afterwards, lest it would wipe right off. Probably need more time than a few google searches. Haven't even investigated acquiring some raw MgF2 material. Not interested in "multi-coating". An old Rollei lens springs to mind. As far as the OP's question, I believe this thread has already provided the answers with respect to retaining lens curvature.
     
  22. Tom1956

    Tom1956 Inactive

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  23. lxdude

    lxdude Member

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    Ya'll just need a laser to heat it, that's all.:happy: :wink:
     
  24. Tom1956

    Tom1956 Inactive

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    Seriously, I would think that google searches on the subject of DIY lens coating would probably turn up my handle in conjunction with the subject; now, and at earlier dates. I was bound to purssue this matter to a reasonable conclusion in my own mind. I believe it can be done, but I also believe that it is also a good way to die as a pauper from spending too much of one's life on such hobbies. In our lives, time is money, and like it or not the best way is still to send the lens off to the big boys who do this sort of thing for a living. Even if someone like me was successful in this wheel re-invention, there is still the matter of the experience. Like any technical field, experience only comes with years. So now I can comfortably close the matter. Thanks, guys.
     
  25. lxdude

    lxdude Member

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    If you have really old glass, you could artificially age it chemically, as the first attempts at anti-reflection coating were done, after it was observed that glass which had formed a tarnish had better transmission and contrast.
    That could be fun, with no big investment.
     
  26. illumiquest

    illumiquest Member

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    I really don't want to coat the glass. I want to get rid of polish marks and signs of 165 years of use...