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Fred_the_oyster
Tinkerer
Username: Fred_the_oyster
Post Number: 14
Registered: 11-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, April 09, 2009 - 03:43 am: | 
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I have the problem described in the message title! The rangefinder spot is clearly visible but about half the viewfinder area is black. I presume it is caused by degradation of the cement used in the viewfinder/rangefinder prism; if so could anyone suggest a method by which it could be repaired?
Many thanks -
Jim |
Gez
Tinkerer
Username: Gez
Post Number: 61
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, April 09, 2009 - 04:25 am: |
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If the prism assembly is secured by Canada Balsam you can wash the dried material away with acetone and cement the pieces together with glass glue. The adhesive stays liquid until cured with UV light, bright sunlight does the trick, but it may be worth practising on spare bits of glass.
Don't bother getting the 'specialist' Norland stuff. Perfectly adequate stuff is available in DIY stores. |
Fred_the_oyster
Tinkerer
Username: Fred_the_oyster
Post Number: 15
Registered: 11-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, April 09, 2009 - 01:44 pm: |
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Thanks for the advice, I'll give that a go - I don't fancy my chances of a 'parts' camera! Cheers |
Glenn
Tinkerer
Username: Glenn
Post Number: 543
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, April 09, 2009 - 02:16 pm: |
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Just remember that modern UV cured adhesives are not reversible, so any error could have serious consequences. In many cases you do not need to bond the pieces of glass together with adhesive on the mating faces. I have 'repaired' a number of prisms by holding the parts together with small pieces of adhesive tape applied to the outer edges - all the repairs have stood the test of time, but can be easily redone should the need arise. |
Gez
Tinkerer
Username: Gez
Post Number: 62
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, April 09, 2009 - 02:35 pm: |
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UV cured methacrylate adhesives CAN be removed simply by soaking the parts for 2 or 3 days in dichloromethane solvent. This solvent is not an over the counter item but can be obtained fairly easily. |
Fred_the_oyster
Tinkerer
Username: Fred_the_oyster
Post Number: 16
Registered: 11-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, April 09, 2009 - 03:02 pm: |
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It appears that the two sections of the prism are secured - or possibly reinforced - with circular pieces of some thin metal, top and bottom.
I understand from what I've read about the Contax rangefinder - and I presume this to be similar - that there's a vacuum-deposited layer of gold on one of the prism surfaces?
I trust that my final question isn't facetious, but the answer would certainly interest me - I seem to remember from secondary school physics that UV doesn't readily pass through glass but is converted to infrared en route. I presume the glass is UV transparent to a sufficient extent to cure the adhesive?
Thanks to all for your help, greatly appreciated. |
Gez
Tinkerer
Username: Gez
Post Number: 64
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, April 09, 2009 - 03:52 pm: |
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Your question isn't facetious and you are correct, glass does not transmit much UV radiation below about 340nm in wavelength, but enough at longer wavelengths gets through.
If you require vacuum deposition work, there is a firm in London called Scientific Mirrors that does 'silvering' of prisms, but they are pretty expensive. |
