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#1 clearwaterdave

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Posted 26 September 2020 - 06:45 AM

Hello.,I have a 90mm Meade objective that "looks" like it is an oil spaced?., And it looks like the lens has been squeezed and the oil was forced to the edges.,One edge has the oil looking like it settled in this side.,and the other edge the oil looks like it's trying drip back to the center.,

  The lense's are cemented together.,Any idea's.,It makes rainbows flash in the fov when looking at the moon.,You can see it along the edge in the reflection.,that rainbow and blue/purple line is the "oil" on the inside,

  Any idea's on what to do.,Thanks,.

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Edited by clearwaterdave, 26 September 2020 - 06:49 AM.


#2 TOMDEY

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Posted 26 September 2020 - 08:34 AM

That's most likely Balsam or Norland Cement or similar --- not oil. Three inches (75mm) is just about the limit that a cemented crown/flint interface can tolerate without eventually failing. What you are seeing there is the failure of the cement. Very common... actually expected with too-big cemented doublets. It seems to be environmental (temperature) cycling that causes the failure. The crown and flint elements have different Coefficients if Thermal Expansion. Cement them together at one temp (typically ~room temp~) and then use them outside... and the stress at that interface is huge!    Tom


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#3 clearwaterdave

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Posted 26 September 2020 - 08:50 AM

That's most likely Balsam or Norland Cement or similar --- not oil. Three inches (75mm) is just about the limit that a cemented crown/flint interface can tolerate without eventually failing. What you are seeing there is the failure of the cement. Very common... actually expected with too-big cemented doublets. It seems to be environmental (temperature) cycling that causes the failure. The crown and flint elements have different Coefficients if Thermal Expansion. Cement them together at one temp (typically ~room temp~) and then use them outside... and the stress at that interface is huge!    Tom

Is there a way to clean it out..And put humpty back together again.,?.,thank you.,


Edited by clearwaterdave, 26 September 2020 - 08:53 AM.


#4 Jon Isaacs

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Posted 26 September 2020 - 09:23 AM

Is there a way to clean it out..And put humpty back together again.,?.,thank you.,

 

There are some threads on the subject in the classics forum.  Here's a thread:

 

https://www.cloudyni...s/?hl= cemented

 

Some have used oil when reassembling the lens.

 

Jon


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#5 clearwaterdave

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Posted 26 September 2020 - 03:53 PM

Thank you Jon.,I will try the boil an oil method.,lol.,Though I will try to steam it rather than submerse it.,wish me luck.,

  The pics and description of the glue breaking down doesn't sound or look like my lens.,mine is clear except where the substance has settled around the edge.,no yellow or haze or flaking.,it looks like oil with the rainbow color in it,.On one edge it looks like it's dripping.,and on the other it looks like it settled along the bottom edge about 3/8"wide/deep by 3+"long.,and the center of the lens is clear.

  I will get them apart and read some more about oiling between and give it a go.,best2all


Edited by clearwaterdave, 26 September 2020 - 03:54 PM.


#6 Oregon-raybender

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Posted 26 September 2020 - 03:58 PM

That is from temp issue. If was on one side then it would have been a hit. This should not be common for small

ones, but larger than 80mm seems to be an issue. Not enough glue on the edge, too thin, so it pulls apart.

 

Not sure what it's glued with, this is a tough one. How to take it apart.

I doubt that is UV, since it's a very hard cement to take apart, if one can? So my guess it's Balsam, but why would they use it?

 

Check other forums, this will not be easy, the process requires heat, removal of the cement, etc... It can get scary.

BTW, try the freezer first, it's better than oil on the first try, safer for you.

 

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Edited by Oregon-raybender, 26 September 2020 - 03:59 PM.

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#7 TOMDEY

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Posted 26 September 2020 - 05:23 PM

Balsam was common before the various UV-cure cements came along. It's essentially fancy distilled tree-sap and can be dissolved with turpentine, which actually comes from the same/similar source, so they're entirely miscible. I saw people using it at work (Bausch & Lomb long long ago) but I didn't pay much attention to exactly what they were doing. It was also used on microscope cover slips... I think because it was less likely to attack e.g. microtomed tissue specimens.

 

Caution... be sure to boldly label the Balsam Cement and Maple Syrup bottles. else get them confused.    Tom


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#8 davidc135

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Posted 26 September 2020 - 06:28 PM

Is there any cement on the edge that could be tested with turps to check if it's balsam. Doubt if it is but if so would it not soften with heat and the lenses easily slide apart? You could put the objective in a cloth sling in warm water and then bring the temperature up slowly.

For uv cured cement a member mentioned using a blow torch! Not for the faint hearted... or was it a joke?   David



#9 ed_turco

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Posted 29 September 2020 - 09:09 AM

Balsam was common before the various UV-cure cements came along. It's essentially fancy distilled tree-sap and can be dissolved with turpentine, which actually comes from the same/similar source, so they're entirely miscible. I saw people using it at work (Bausch & Lomb long long ago) but I didn't pay much attention to exactly what they were doing. It was also used on microscope cover slips... I think because it was less likely to attack e.g. microtomed tissue specimens.

 

Caution... be sure to boldly label the Balsam Cement and Maple Syrup bottles. else get them confused.    Tom

Cementing a lens with maple syrup would be a sweet idea.

.
 



#10 jtsenghas

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Posted 29 September 2020 - 11:09 AM

Cementing a lens with maple syrup would be a sweet idea.

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#11 clearwaterdave

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Posted 29 September 2020 - 11:48 AM

I got syrup.,lol.,I got balsam sap.,an superglue.,but first I need to get them apart.,haven't tried yet.I marked the edge and set it aside.,waiting for idea's.,thinking it through.,the mount is coming tomorrow.,more to play with.,



#12 dan_h

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Posted 29 September 2020 - 01:03 PM

Did Meade ever produce a cemented 90mm objective or is this one originally an air spaced unit that someone has tinkered with and attempted to improve by cementing it? 

 

dan



#13 Steve Dodds

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Posted 29 September 2020 - 02:46 PM

The only way you can cement a lens is if the inner 2 radiuses match. or are at least very close.  There are a lot more design options if you can make the radiuses anything you want, but then you will have to airspace.  So I would say no you can't take an airspaced lens and cement it.


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#14 DAVIDG

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Posted 29 September 2020 - 03:23 PM

The only way you can cement a lens is if the inner 2 radiuses match. or are at least very close.  There are a lot more design options if you can make the radiuses anything you want, but then you will have to airspace.  So I would say no you can't take an airspaced lens and cement it.

 Correct. The air space is an  optical element just like the glass. You have light going from a high refractive index to a low one , which is  air, and then from air back into a high refractive material. The light path follow Snell's Law when it comes to exiting and entering materials of different refractive index. Take out the air and the light path changes.  The designer  uses air space as refractive element and as pointed out you can now have difference in the two inner radii. This gives more degrees of freedom and so the ability to better correct the lens. 

 

             - Dave 



#15 clearwaterdave

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Posted 29 September 2020 - 06:22 PM

This is cemented.,and it looks like it has oil in between.,viewing in the daytime looked ok with a quick glance.,but didn't like lookin at luna at low power.,rainbow reflection all over.,I think I can get it apart and clean it., but then I'm not sure what to do.,

  All help welcomed.,This scope was a freebee for me to play with.,I know nothing of it's history other than it was in N.C. If I can make it work it will be a loaner or a give away.,cheers.,

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#16 davidc135

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Posted 29 September 2020 - 08:30 PM

After cleaning off the cement I'd check first to see how it works air spaced. Check to ensure that the 2 lenses are the right way round. R2 and 3 could be contacted to see if Newton's fringes show. If so, the objective could be temporarily oiled with veg. oil and the moon etc viewed.

 

David




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