28 replies to this topic

[DAVIDG]

  If you work on vintage telescopes long enough sooner or later you're going to come across a lens that the cement  between the elements has gone bad  and is hazy and the element have started to  separate.  The typical cement that was used was Canada Balsam which will soften at about 150° F so you can heat the lens in water or an oven to get them apart. Once apart you then clean the elements with solvent.  Now it is time to recement the elements.. In the past one had to dissolve the Canada Balsam in Xylene and heat it up, then quickly cement the elements together before it harden without getting any bubbles and getting  the elements centered.  So it could take a couple of   times to  get things right if you weren't skilled in the process.

  Here is were modern UV curable optical cement makes the job easy.  I 'm restoring a pair of  SARD 7x50mm WWII binoculars and the cement in one of the lenses in the eyepiece had failed. In my lab we had an expired bottle of Norland 63 cement that couldn't be used for our projects and was getting tossed but when I tested it, it was still working. In my lab I have 365 nm UV light source that is what the cement works best with but I have also used a UV LED flashlight which worked but just took longer to cure the cement which was about 15 minutes. What makes this material easy to use is that until it is exposed to UV light you can move the elements around and make sure they are centered and there are no air bubbles

    So here are some pictures  showing the process. The first is the defective lens as  it looked when I removed it  from the binoculars. You can see that  it is yellowed and when you looked thru it, it was very hazy.

 

                    - Dave 

 

 

Edited by DAVIDG, 12 May 2019 - 10:58 PM.

[DAVIDG]

 I placed the lens in my toaster oven and heated the lens until the old cement soften then used two wooden sticks to push them apart. Then cleaned the old cement off with some solvent.

 

                         - Dave 

 

[DAVIDG]

  The next step was to make sure the elements were clean and I  added  a drop of cement to the concave element. Then the convex element was placed on top of the concave one being sure that the cement reached the edges of the elements with no air bubbles.

   

                   - Dave 

Edited by DAVIDG, 13 May 2019 - 11:06 AM.

[DAVIDG]

 I used a metal clamp as  a V block  that  I used to  push the elements against to make sure that they were centered. Then the lens was  placed under the UV lamp. It took about 5 minutes for the cement to cure but I left the lens under the lamp for about 15 minutes just to be sure the cement was well cured.

 

              - Dave 

 

 

                          

[DAVIDG]

 Here is the finished lens. It is now crystal clear and ready to be reinstalled. A bottle of Norland cement cost about $35 but  you can get a small quantities on Ebay  for under $10 which is enough to do a small lens or two. You can also use sunlight as the UV source.

 

                     - Dave 

 

 

 

 

 

[starman876]

I would have never thought of using a toaster. I have boiled lenses the get them apart because the glue starting going bad.  But a toaster?????   I would have thought the lenses would have gotten to hot and cracked.   

[Terra Nova]

Great information here! Thanks Dave!

[DAVIDG]

I would have never thought of using a toaster. I have boiled lenses the get them apart because the glue starting going bad.  But a toaster?????   I would have thought the lenses would have gotten to hot and cracked.   

 Note I said toaster oven, the type were you can set the temperature and select Bake or  Broil or Toast. I set the oven  on Bake at 200 F and watch the lens. You can see the cement soften then turn off the heat, keep the lens in the oven and use two stick to push the elements apart then close the door and let everything cool back down before taking the elements out.

 

                        - Dave 

[starman876]

 Note I said toaster oven, the type were you can set the temperature and select Bake or  Broil or Toast. I set the oven  on Bake at 200 F and watch the lens. You can see the cement soften then turn off the heat, keep the lens in the oven and use two stick to push the elements apart then close the door and let everything cool back down before taking the elements out.

 

                        - Dave 

Good point. Missed the oven part .

I have only separated a couple of glued lenses and really do not want to do anymore.  It just worries me the whole time I am going to crack the lens.  However, this method seems a little easier trying to do it in a pot of boiling water.

[markb]

Another alternative for those afraid to commit on re-cementing smaller binocular lenses is an oil film.

 

I have a failing pair of Flak 80's for which I will fully employ DavidG's method, thanks (x100, given everything I have been learning in the last two months, starting with your Dynamax posts and going through your many optical narratives and comments to others!).

 

Before these UV products were available I was dead afraid to re-cement my Dad's great 6x35 Steinheils, after he finally managed to split a cemented objective. He sure got his money's worth on those 1952 binoculars.

 

I head read  (pre-internet) about oil being used, and gave it a try with 3-in-1 or 10-30 motor oil, one drop on the cleaned flint, carefully lower the crown, and reinstall, carefully. Despite paying no head to the refractive index of the oil, it worked wonderfully well, holding up for 15+ years.

 

But I am using DavidG's method this time.  I already bought matched machinst's V-blocks on EB for about $30. Other oft suggested centering methods include using surplus binocular prisms, still available cheaply, to butt up against the lenses to keep them centered.

[KentTolley]

David, you are braver than  me as well as more knowledgable.  I just sent a Orthostar 26.6mm ep to Gary Suddarth binocular repair.  The  cement had failed and there was a cloud in the image and I was afraid to fix it myself.  Thank you for detailing your process so next time I can do it myself.  

[DAVIDG]

 I'm continuing to restore the Sard 7 x 50mm binoculars. One of the 50mm objectives had the elements separating as well.  If you haven't recemented a lens before, one might look at this one and think  it is not easily repairable but as I'll show again it is easy repaired with excellent results. Here again are  pictures of how I repaired them. 

   First this  what the lens looked like when I removed it , yellowed with many cracks in the Canada Balsam cement. Pretty ugly !

 

                   - Dave 

 

 

Edited by DAVIDG, 15 May 2019 - 08:34 AM.

[DAVIDG]

 Next step is into my toaster oven to heat it up to soften the cement and separate the element.  Set the heat at low and watch the lens as it heats up. You'll see the change in appearance when the the cement softens. Then I reach in with two sticks and push the elements apart. Turn off the heat, close the door and let the elements cool down slowly to room temperature.

 

                       - Dave 

 

Edited by DAVIDG, 15 May 2019 - 08:35 AM.

[DAVIDG]

 Once the elements have cooled off, I clean the old cement off with solvent. I spray the lens with carburetor cleaner which is a mixture of solvents and  it easily dissolves the old cement. The elements are then washed with soap and water and made sure they are perfectly clean. 

  The flint element is placed with the concave surface facing up and a couple of drops of the UV cement is added to the middle. Then the convex element is placed on top and light pressure is applied to  the top element to evenly push the cements out to the edges and remove any air bubbles.

   Then the assemble is pushed up against a clamp to make sure the elements are centered and then exposed to UV light for about 15 minutes.

 

                          - Dave

 

Edited by DAVIDG, 15 May 2019 - 08:36 AM.

[DAVIDG]

 Here is the finished lens. Back to being clear and ready to be installed in the binoculars. So I hope this shows that just because a lens looks ugly it is not difficult to repair.

 

                        - Dave 

 

[semiosteve]

Great thread...I have an original Masuyama 35mm EP with a clouded cement issue.

 

I got it dissolved via the boiling water technique but lost the marks I had made for lining up the elements again.

 

So I did not re-cement it yet, but reassembled it as best I could. After a bit of fiddling I get pretty good images without recementing. Stars come to focus sharply but in-focus and out-focus looks funky.

 

So I am hesitant to recement as I don't know whether I will make things better or worse.

 

But I am very pleased to read this thread and know there is a more manageable means to do so if I change my mind.

 

Great info!

[KentTolley]

This should be added to the sticky post "Useful classic scope links"

[DAVIDG]

 I doubt that you need any rotational alignment of the two pieces of your eyepiece lens. They just centered and glued them when it was made. Centering was the critical part and that is simple to do with  a V block.

   What  re-cementing them will do is remove the air space since the optics were design not have one.  That will improve the contrast a bit since those surfaces are not coated and make the optical correction in theory a bit better since the optics were designed to have no air space.

  An air space is an optical element just like glass. The light is traveling from the glass which has higher refractive index into air which is  lower in refractive index then back into another piece of glass that has higher refractive index. When cemented the light travels from one glass element into the other so it interacting with only those two refractive indexes not three when you have an air gap.

   So you don't want to oil or cement a lens that was designed to be air spaced since again that air space is just as important has the pieces of glass in the over all optical correction. 

 

                  - Dave 

[markb]

Hi David

 

Your second set of photos clarified many of the essentially points on the procedure, thanks.

 

I will brave the toaster oven to split my 'snowflaked' 70 year old flak 80s.

 

And thanks again for your extensive refiguring and testing threads.

 

Do you have a opinion on use of LOCA cements, alleged to be less 'forever' than traditional UV setting epoxies?

 

And is there a home-viable way to decement UV epoxied doublets? 

[DAVIDG]

 Hi Mark,

   I have a pair of Flaks that I need to re-cemented as well but the front surfaces are sleeked so I need to repolish them before re-cementing them.

   I have experience with only the Norland UV cements.  Norland has some recommendations on how to separate a lens that was cemented with their products https://www.norlandp...separating.html

   As a chemist/engineer that would be no problem for me but I have an industrial lab to do it in  but for someone at home the solvents they recommend are nasty stuff. So I would not recommend it. 

    I have never done this but I know Ed Jones who is  a Master Optician has stated what he does is pass the flame from a propane torch over the lens and the thermal shock will pop them apart. 

  In my experience when a lens  has separated  it has usually been cemented with Canada Balsam or some type of cement that can be heated up in the 200 to 250F range to soften it and get the elements apart.

 

                      - Dave 

[Chuck Hards]

Here's my old thread on de-cementing a 5" achromat.   Must have been an early man-made optical cement, it definitely wasn't balsam.

 

LINK

[markb]

Yep, methylene chloride and ammonium hydroxide, uber-nasty. I had read in my watchmaking adventures that this works on UV cured crystal adhesives but it is nasty stuff on its own.

 

I have only encountered balsam separations in my very very limited experience; one nice thing is they can be split, easily thanks to your tutorial.  I had heard temps to 350 were needed (not wholeheartedly believing it), and the bubble bouncing effect of a lens in boiling water was not something I was anxious to try (but I would have used a silicone pad at the bottom of the pot).

 

The toaster oven method, with temps to use, is golden.

 

Thanks for the answers.

 

If I try the LOCA adhesives on my old Steinheils I will post or PM, but that is waaaay down on the pre-move to AZ list. If I split the flaks, I will preliminarily join them with an optical oil film. Too scary to risk those old, irreplaceable, f3.5 280mm lenses o  a permanent adhesive. I was able to retrofit one pair with 300m fl lenses with extended cells, but still...

[Gordon Rayner]

Do not   use the hot water separation method on WW II German  optics.  They used cryolite  ( sodium aluminum  flouride),  rather than MgF2,  in many /most/ all instances.   It  gives higher light transmission  than magnesium flouride.  There may have been  easier heating or vacuum considerations  for  production

 

Cryolite  is soluble in  hot or warm water.   I lost some of the coating on the rear  surface   of  the objective  of  a 10 x 80.   I mixed   DMSO  with water,  and immersed the doublet  in the mixture,  placed  it  below the windshield  of   a closed,  sun-exposed  car.   Some of the  cryolite  dissolved. 

 

DMSO  works  for immersion,  but can be slow.   It is available  as horse  liniment , and   reputedly helps  human  arthritis  victims.  Methylene chloride  has a tiny molecule, for  improved  bond  breaking,  and was or is  a key ingredient in paint strippers.   Xylene  or  toluene  are good for  balsam  cement  separation.   But those aromatics  and chlorinated  solvents  are no longer  available at Home Depot  etc.  in California,  for smog reduction  or other  Cal-OSHA    reasons.

 

I have  been  chicken  about  using   many heat methods.  What  happens  when one opens  the toaster oven, etc.  , to push apart the elements.?   The room  is   a much lower temperature, presumably, implying  thermal shock on the glasses.   Solvent immersion   is/was   preferred,  unless one is in a hurry  or  cannot buy  the solvents.

 

I have two Navy (  rotatable  lens chuck/  opposed   target  collimator -  reticled  inspection  telescope pairs )  instruments,  and have or had  several  similar Pearl brand   Japanese  lens  cementing  instruments.   V-blocks  give  the same results,  in my experience,  for use cementing  well  ground  centered  elements. 

 

I have used Norland  NOA  61  adhesive,  cured with UV   blacklights  from   mineral  displays   or disco era  illumination.

 

 

 

 

 

 

 

       

[Earthbound1]

Great thread!

[starman876]

Awesome thread Dave.  We should take all your educational posts and make a book out of it.