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Separation of a doublet and re-cementing

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#1 Gene7

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Posted 05 October 2011 - 06:32 PM

Originally this 2 inch doublet lens was cloudy with some voids. Various means of separating it had been tried, such as heating and common solvents, all without success. See Martin post, “ Seperating a finder scope doublet” this Forum.

Since I had separated and re-cemented doublets before I volunteered to do it. Of course the edges of both elements were marked with a lead pencil as to cemented sides and rotational position.

After gentile boiling for 10 minutes in Summers Optical De-cementing Agent fluid the lens separated. A sticky film was left on all parts of the lens, which cleaned up with the suggested acetone solvent. I assume the cement was balsam. Previous lenses I had separated produced a thin clear sheet of cement that mostly broke up into small flakes. See photo of separation procedure:
http://farm7.static....7600499c7_z.jpg


The doublet was re-cemented using Summers Optical Lens Bond type UV 74 polyester cement. This is a UV activated polyester cement. The procedure is to apply the proper amount of cement to the concave side of the doublet, add the convex half, and manipulate it a bit so the cement spreads to cover the entire lens. Bubbles must be excluded, a slow orbiting movement (do not spin) may be used and a pressure of only about the lens weight can be applied. The consistency of the cement is about like honey. See photo of element fixturing:
http://farm7.static...._fe37398ab4.jpg


The doublet was exposed to the UV rays of a suntan lamp, the older type with a clear quartz discharge tube. Yes, lots of heat. The doublet must be supported accurately without any strain on the elements. I used three hex wrench sockets for triangular positioning. See photo. The procedure is to cause an initial set in the cement, inspect it for position, and clean off any edge excess cement, and then do a final cure. A minute was sufficient for an initial cure, but I went 10 minutes. The edges were cleaned with a razor blade and a cloth only slightly damp with acetone. Final cure was then made for 60 minutes. To avoid overheating the lens it was allowed to cool several times during the cure. The anti-reflective coatings survived intact.
See photo of U.V. bonding cure exposure:
http://farm7.static....beb97c925_z.jpg


Before the cementing was attempted some glass squares the size of the lens were cut and cemented. This gave the vital information of how effective the UV lamp was and how many drops of cement to use. ( I used 6 drops from an eye dropper) Final cure is to be 60 times the exposure it takes for an initial cement set.

Since my lamp was designed to be used in the horizontal position (to avoid overheating) a fixture with a 45 degree first surface mirror was assembled. The light was thus reflected to the doublet, which must be laid out flat. A piece of foil under the lens doubles the exposure.

My comments are only to supplement the information supplied by the Summers Optical Co. for both separation and cementing. Do read http://www.optical-cement.com/ before attempting. Use the separation solvent only out doors. Protect the eyes from UV. Store Lens Bond in the frig. between uses, but let warm to near room temperature before opening to protect from moisture condensation. Gene

#2 Pinbout

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Posted 05 October 2011 - 06:50 PM

very cool, just the other day I was wondering about an old 4x5 lens I have that has cement problems and if I could recement it. I'll have to look into to it further.

#3 mercedes_sl1970

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Posted 06 October 2011 - 05:17 AM

Thanks for posting about your experience. I went through this a few years ago but just heated the (eyepiece) lens in water and then re-glued using a UV curing glue - midday summer sunlight. I didn't know there was a specific de-cementing agent.

Thanks again.

Andrew

#4 Mark Harry

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Posted 06 October 2011 - 06:50 AM

Gene, I'd like to ask about a comparison to the 2 basic cenents- the one like you describe here, and balsam. For a first timer, which would have less issues that would affect the end results? I have an upcoming 2.75" doublet I'm making that I'd like to cement. (don't have a UV source)
Mark

#5 Benach

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Posted 06 October 2011 - 07:03 AM

Mark: is New England always cloudy? BTW, I wouldn't cement just any doublet. Sometimes the inner radii do not match or the types of glass do not match because of the large thermal expansion. This might cause stress in the optics.

#6 Mark Harry

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Posted 06 October 2011 - 11:31 AM

Lately, -YES-! (actually, this week's to be clear, with a -WAXING- moon, as is typical)
****
I have a project going; a Littrow, with 3 radii the same. Only R4 is different. I designed it with zero spacing, and intended to cement it if the mfr works out. It's only 2.75" diameter, so should work, I assume.
Positive crown is polished out.
Mark

#7 Gene7

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Posted 06 October 2011 - 03:08 PM

I am not an expert Mark, but a couple of things come to mind. Although the lens I just separated appeared to be balsum, since all parts of the lens was sticky afterward, it was not possible to separate with common solvents. I believe it was because it was old.

I believe fresh balsum would separate easily. That said, I would worry about the aleignment of the pair after cementing, one element could slide and the optical centers not be aleigned. Of course you do not want to clamp a lens to cause distortion.

So, take your choice if you think you might want to do more work on the lens after testing. Perhaps an oil could be used for even easier separation for test.

Perhaps e-bay would be a source for an old type sun-lamp. Sounds like something a person might want to sell after a dose of cancer! I did try using a flourscent UV germicidal lamp, a T5 4 watt. Not effective at all. I have heard that a black light flourscent is effective, but have not tested one. Other people have said that sunlight is not effective enough, although an above poster has had some benefit. Will see if I can find my black light to test. Gene

#8 Gene7

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Posted 06 October 2011 - 03:17 PM

Benach, you are correct that only doublets that were designed to be cemented should be. If a person has a lens he knows should be cemented you can usually determine which side of the double convex by fitting it to the concave section with very small water droplets over the lens. You can tell the best fit to cement by seeing which side matches the best. If they are identical, who cares.

Another tip, if a separated pair has only one side optically coated you can bet the uncoated sides should be cemented. Gene

#9 ccs_hello

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Posted 06 October 2011 - 09:00 PM

Gene7,

Thanks for the writeup. I'll have to give it a try on my Canon camera lenses.

Clear Skies!

ccs_hello

#10 Gene7

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Posted 07 October 2011 - 06:59 PM

Major Cement Curing Improvement

I found that a 13 watt standard base compact fluorescent Black Light bulb is very effective in curing the UV activated lens cement. More effective than the 475 watt sun lamp I started with. I paid less than $5.00 at the Depot. The initial cure for securing the elements takes place in less than a minute, I did not investigate how much shorter.

I had investigated a UV germicidal lamp and a high powered UV LED. Neither were effective in promoting a cure of the polyester cement.
Gene

#11 dcoyle

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Posted 08 October 2011 - 09:45 AM

Gene7,

Thanks very much for posting this info.

Dan

#12 Dan McConaughy

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Posted 22 October 2011 - 08:40 AM

Did you check the wedge after recementing? I'd be concerned that there might be wedge or isn't that an issue with lens cement?

#13 Stipe Vladova

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Posted 12 November 2011 - 02:58 PM

Hi to everyone. I purchased 60mm f15 objective lens from eBay and discovered that its cell is poor and of molded plastic. It is an airspaced achromat. Due the minor decentering or misalignment the axial coma, axial astigmatism and lateral colors occure. I tried to tape the lenses together and that hold about a day and misalignment occured again. That objective has no spherical, astigmatism, no wedge or anything, but a tiny turned edge (R3 curve on the flint). The image quality was similar to Zeiss telementor so I decided to build a similar scope and I am also the fan of small long refractors. Today I put the objective on the autocollimation test against my 12" paraboloidal mirror, centered it, taped together and when I saw that it gives good image I put the objective in its cell and tube, looked again after - the horrible lateral colors and axial coma :confused: I know that lateral colors can occur if one of the lens is decentered (if no wedge of course) or slided aside a little. Then I decided to cement it, but I am not pretty sure if it is recommended since the R2 is slightly more curved than R3. Can I cement the Fraunhofer objective anyway. In my local glassworkshop there can I order some UV cement. Should I try?

#14 Ed Holland

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Posted 12 November 2011 - 03:59 PM

To Stype,

Try to reassemble in the cell with spacers between the elements, and something to retain position at the edges to prevent sideways movement. Then use a trick I learned fixing my scope. Put another set of spacer pads under the retaining ring at the same positions as the spacers between the lenses. Card or felt is good for this. As long as the pads are right over the spacers they will hold the elements properly without pinching. Tighten the retaining ring only gently. This worked perfectly on my 5" scope which would not hold properly beforehand

Re optical adhesives, the UV curable resins have photoinitiators that usually require pretty strong UV in the vicinity of 365nm which is a intense line available from a mercury discharge lamp. Any other source needs to have reasonable energy in this band

#15 Stipe Vladova

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Posted 12 November 2011 - 05:40 PM

I think that the new cell must be machined on a lathe, the durable hard plastic or aluminum must be used, but I dont have lathe and nobody here wants to machine telescope parts because they think that is complicated. Here in Croatia are few us who make telescopes and ATM is rare here. The original cell is 1,5mm wider inside than lens so it is the cause lens to slide sideways.

#16 Gene7

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Posted 12 November 2011 - 06:17 PM

I see no problem in cementing three shims inside a cell to properly space any lens. DO NOT cement a lens that is supposed to be air spaced. It will be junk until you un-do the damage.

For lens UV cement the Halloween black light compact fluorescent works best and is safe. I have tried a mercury vapor lamp and a sun lamp and they do not work well.

As for making a new plastic cell you can use a rotary file in a drill press if you take care. Do not let the chuck pop off. I have used larger plastic pipe fittings. Gene

#17 Ed Holland

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Posted 12 November 2011 - 09:42 PM

I think that the new cell must be machined on a lathe, the durable hard plastic or aluminum must be used, but I dont have lathe and nobody here wants to machine telescope parts because they think that is complicated. Here in Croatia are few us who make telescopes and ATM is rare here. The original cell is 1,5mm wider inside than lens so it is the cause lens to slide sideways.



May I first suggest that you place something at the edges of the lens to prevent them from moving in the cell? Pieces of card or thin plastic can be used to fill the gap and hold the glass in the very centre of the cell. It is worth a try and can be done easily - no lathe required :)

Ed

#18 Stipe Vladova

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Posted 07 January 2012 - 01:05 PM

Now I discovered that slight decentering in exact position of about .3mm is needed to achieve image with no lateral colors. That means that one of the lenses has a tiny wedge. This requires UV cement to hold in that exact position if I want the image with no lateral colors. The colors are not visible at medium magnification, but at about 80x is clearly visible. There is one portion of field of view that has no lateral colors, but on the opposite side of field the colors are strongest reducing the contrast and sharpness. I put the oil between several times and there was less longitudinal chromatic what impressed me, but oil can't hold the lenses in exactly desired fixed position.


Second thing is that Ihave one 73/300mm binocular objective that is cemented with epoxy. It is a bit overcorrected so it means that I must separate them and increase air gap to achieve desired spherical correction. Now I don't know how to separate the elements. Has anyone some experience on that?

Thanks.

#19 MKV

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Posted 07 January 2012 - 01:10 PM

Acetone might help separate epoxied lenses.

#20 Stipe Vladova

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Posted 07 January 2012 - 01:18 PM

I left the lens in a plastic container with acetone for a few days and nothing happened. I have successfuly separated earlier some smaller lenses with sharp knife, but that were 30mm lenses. This is much bigger and may break it if I try.

#21 Gordon Rayner

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Posted 07 January 2012 - 03:10 PM

Try lacquer thinner, xylene (xylol), toluene(toluol), DMSO ( dimethyl sulfoxide, a by-product of the pulp and paper industry, and used as horse ( or human, for arthritis ) liniment, so get it at veterinary supplies shops). Those solvents are listed in the order of increasing strength, from experience. But there are many variables, so you might have a different heirarchy of solvent power.

If all else fails, immerse the lens in a used steel food can with cooking oil. To avoid scratches, pad around and under the lens with paper towels, toilet paper, nose wiping tissue, etc. Then put that into a cold oven. Heat to about 375-400 deg. Fahrenheit, or until the oil is barely smoking ( or until your wife tells you to stop stinking up the house interior). Let everything cool to room temperature.
That treatment will separate , or at least greatly weaken, the bonds, either from direct thermal attack on the cement, or/and differential thermal expansion of the lens components and the cement. Further attack with solvents might finish the job, if separation is not yet complete(BUT NO SOLVENTS IN AN OVEN).

#22 Stipe Vladova

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Posted 07 January 2012 - 04:52 PM

Now I put the achromat in the lacqueer thinner and will wait several days to see any effect.

#23 Gene7

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Posted 07 January 2012 - 08:37 PM

You are not reading my material. You MUST use the decementing agent and links posted on the first page of this link. Do not pry and and/or over heat the lens, it will not work. Get back to me if you have problems, but follow the above instructions with the suggested materials. Gene

#24 Stipe Vladova

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Posted 07 January 2012 - 10:21 PM

I must first get the decementing agent. Here in Croatia unlike there in US are stuff hard to find.

#25 Stipe Vladova

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Posted 10 January 2012 - 10:17 AM

Try lacquer thinner, xylene (xylol), toluene(toluol), DMSO ( dimethyl sulfoxide, a by-product of the pulp and paper industry, and used as horse ( or human, for arthritis ) liniment, so get it at veterinary supplies shops). Those solvents are listed in the order of increasing strength, from experience. But there are many variables, so you might have a different heirarchy of solvent power.

If all else fails, immerse the lens in a used steel food can with cooking oil. To avoid scratches, pad around and under the lens with paper towels, toilet paper, nose wiping tissue, etc. Then put that into a cold oven. Heat to about 375-400 deg. Fahrenheit, or until the oil is barely smoking ( or until your wife tells you to stop stinking up the house interior). Let everything cool to room temperature.
That treatment will separate , or at least greatly weaken, the bonds, either from direct thermal attack on the cement, or/and differential thermal expansion of the lens components and the cement. Further attack with solvents might finish the job, if separation is not yet complete(BUT NO SOLVENTS IN AN OVEN).


I just separated the doublet by heating in vegetable oil in the pot, after visible sign of full separation I put the pot aside to slowly cool down, but when cooled down the flint cracked to the half of it's diameter. :bawling: I was really dissapointed and it looked as the child playing with plastic tools trying to unscrew a real metal screw. :( After separation I also tried to clean the remaining epoxy with acetone but with no success. I don't know maybe it is another type of adhesive resistant to acetone :confused: I increased the air gap between the elements as I told to do and get the full spherical correction as I predicted. Crack does not change the image quality, but looks really bad.


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