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Cleaning and repairing a cemented prism

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#1 decep

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Posted 29 May 2020 - 07:19 PM

A few weeks ago, I purchased a Denkmeier Deepsky binoviewer for a too-good-to-be true price on eBay.  Turns out the cement between one of the prism elements had started to turn hazy.  Instead of getting mad, I started investigating repair options.  I figured I would document my journey in attempting to repair the prism.

 

IMG_20200509_123909.jpg

 

IMG_20200513_180133.jpg

 

There are some pretty good resources for information on lens cement issues.  Considering the age of the binoviewer, I felt like there was a good chance the cement was Balsam.  I disassembled the binoviewer and placed the prism in a pot of water (room temperature) to get to boiling temperature.  After a few minutes at boiling temperature, I removed the prism (with tongs) and used some gloves apply some pressure.  I did not have to apply much and the prism elements separated!

 

IMG_20200516_165229.jpg

 

I put the prism back in the pot and turned off the heat to allow them to cool slowly to reduce the thermal stress.

 

IMG_20200529_175226_s.jpg

 

Note:  The spots that look like water spots are just the back of the black paint.

IMG_20200529_175307_s.jpg

 

To be continued....

 


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#2 decep

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Posted 29 May 2020 - 07:24 PM

After using some acetone on some paper towels, the remaining cement came right off.

 

Nice and smooth.

IMG_20200529_183620_s.jpg

 

Unfortunately, it looks like some of the optical coating came off the smaller prism element.  I am hoping that when I apply the new cement, this will not be noticeable.

IMG_20200529_183810_s.jpg


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#3 decep

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Posted 29 May 2020 - 07:32 PM

My plan now is to acquire some Norland Optical adhesive.  The Edmund Optics site has several types in stock.  N61 seems to be a good option.

 

Anybody that has any suggestions, I am willing to listen.  I have never done anything like this before. smile.gif


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#4 jefffed

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Posted 29 May 2020 - 08:33 PM

Seems to be very similar to recementing a doublet. Here's a link to a CN discussion.    Norland 61 is the way to go. I've obtained some small amounts on ebay. For a prism you probably need only a few drops. You can also get a UV nail polish light fixture for not much outlay on ebay that will harden the N61. Let us know how it turns out as there might be some others that this might have happened to.  



#5 jefffed

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Posted 29 May 2020 - 08:35 PM

Sorry the link didn't post. Google "separation-of-a-doublet-and-re-cementing" and you'll see the discussion



#6 jefffed

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Posted 29 May 2020 - 08:38 PM

BTW you could have separated the prism with methylene chloride without heat. Home depot has it in the form of paint brush cleaner



#7 decep

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Posted 30 May 2020 - 12:41 PM

I will check out the other posts.

 

I read about methylene chloride, but I figured I would try the boiling water method first.  I am also trying to preserve the existing blackening paint on the prisms.

 

I have done a lot of research regarding UV lights.  365nm appears to be the sweet spot for curing.  Frankly, I do not trust most of the cheap lights coming out of China (Amazon/eBay) to be of the right wavelength to be effective and I am not really willing to spend a lot of money on something that I can trust (for a one time job).  The sun pumps out plenty of that wavelength, every day, for FREE.



#8 decep

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Posted 23 June 2020 - 07:07 PM

Just an update.  I have managed to re-cement the prism together.

 

IMG_20200623_191328.jpg

 

The boundary is no longer visible.  One or two tiny specs of dust got caught between the elements, but I can only see them if I really look for them.  They will not show up on camera.  They are not at the focal plane so I am hoping they will not be visible.

 

Also, the discoloration on the smaller prism I thought were coating failures from separating the elements seemed to just be stubborn cement that refused to come off with acetone.  I applied a little "elbow grease" while cleaning and it came off.

 

All in all, it was not all that complicated.  I wish I was able to practice before trying it on the actual prism, but it is what it is.


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#9 decep

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Posted 27 June 2020 - 09:50 PM

For anyone that has not seem my other thread, while recementing my prism was an educational experience, it was ultimately pointless.

 

The coating in the second picture of post #2 was the 50/50 coating for the beam splitter.  When I separated prism elements, it also removed that coating which made recementing the existing elements pointless.  [Almost] all of the light is now transmitted straight through instead of being split 50/50.

 

I suppose there was a reason I never found any resources talking about repairing prisms.  :-)

 

post-299052-0-29145900-1590798114.jpg



#10 Ed Jones

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Posted 29 June 2020 - 11:07 PM

You could try silvering a 50/50 coating with spray on silver.  It would be pretty much a trial and error process and not very expensive so you can strip the coating if bad many times as needed. Once you get a good coating cementing will protect it. You can also send it to a coater but that would be expensive. In either case you'll need to remove all of the old coating.




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