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Looking For Cheap Doublets
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PostPosted: Mon Dec 27, 2021 3:12 am    Post subject: Looking For Cheap Doublets Reply with quote

I'm planning to do an experiment with several different kinds of LOCA (Liquid Optically Clear Adhesive, and I'm planning to share the results on the DIY board here), and obviously to do that experiment I need some glass to glue together. So I'm just wondering if anyone here is aware of any cheap lens doublets that I can easily get in a quantity of about 6-10 identical units. They can be used, they can be from adapters, converters, cell phone clip on lenses, anything really, as long as they're made of two cemented glass elements.


PostPosted: Mon Dec 27, 2021 6:16 am    Post subject: Reply with quote

Alternatively, I see I can quite inexpensively obtain single convex and concave lenses. If someone here is able to explain the measurements/terminology necessary to order complementary single elements that will mate flush, I can just go that route.


PostPosted: Mon Dec 27, 2021 7:23 am    Post subject: Reply with quote

https://www.edmundoptics.com/knowledge-center/application-notes/optics/understanding-optical-specifications


PostPosted: Mon Dec 27, 2021 10:28 am    Post subject: Reply with quote

visualopsins wrote:
https://www.edmundoptics.com/knowledge-center/application-notes/optics/understanding-optical-specifications


So two lenses with the same diameter, one with a positive radius of curvature, and one with a corresponding negative radius of curvature would do it? It sounds pretty straightforward, I just want to make sure since this is essentially the first I'm learning anything about optics.


PostPosted: Tue Dec 28, 2021 8:33 am    Post subject: Reply with quote

https://www.surplusshed.com/

They have them in various types and focal lengths ... cheap!


PostPosted: Fri Dec 31, 2021 8:22 am    Post subject: Reply with quote

kds315* wrote:
https://www.surplusshed.com/

They have them in various types and focal lengths ... cheap!


The perfect resource that I never knew existed, thank you!


PostPosted: Sun Jan 02, 2022 12:44 am    Post subject: Reply with quote

Well thanks to kds315* I was able to purchase 6 sets of uncemented achromat doublets, all with a focal length of 105mm, and diameter of 37mm, to practice using adhesive.

I have also purchased 4 different types of LOCA; Norland NOA 61 (from EdmundOptics.com), Xfactor Hydro XFUV10BLK, MMOBIEL TP2500, and Octopus Glue (all three of those from Amazon). I'm probably going to get some actual Canada balsam as well, for comparison, and to practice with separating after adhesion.

My intention is to test all of these LOCA against each other, looking at transparency, viscosity, propensity for bubbles, ease of application, working time, curing time, difficulty of separation/removal, and overall value for money. NOA 61 has been in use for quite awhile as a high quality LOCA for many different applications, so it's probably the adhesive to beat in the test. The three LOCA purchased from Amazon appear primarily designed for cell phone repair (gluing displays to glass digitizers), so it will be interesting to see if they appear suitable for lens repair as well since they're more budget friendly than NOA 61.

My next step is to devise a method for centering the elements before adhesion. My current plan is to use a battery operated turntable, center and attach an LED or laser (red to avoid any UV that might prematurely cure the glues), then center and attach a glass element holder, and use a bamboo chopstick to gently nudge the element in place. Centering all of these components will be achieved by projecting an image of the light source on a piece of paper suspended above the entire setup. When apparent rotation of the projection has ceased it should mean the element's vertex is centered. My intention is to do this as inexpensively as possible, as a demonstration for anyone else thinking of attempting these repairs at home. I haven't decided whether to bother centering the achromat doublets during the LOCA test. It would probably make good practice though.

I'll start a new thread in DIY once I actually begin the project.


PostPosted: Sun Jan 02, 2022 11:52 am    Post subject: Reply with quote

Really looking forward to seeing the results, I would be interested in a feasible DIY centering setup myself.

Interested to see how you devise a turntable setup that allows for light transmission; I always assumed any DIY attempt at that would have too much play/runout in the bearings to be of practical use, as the wobble circle of the transmitted light should be quite small (ca. 0.1mm in the example of a 50mm focal length lens with unity magnification of the optical test setup, allowing for a 10 arc minutes decentering tolerance).

As I understand it, decentering tolerances are usually in the order of some angular minutes, down to some angular seconds for high precision optics (source: Christoph Gerhard, 2018, see below) Very interesting read that book by the way, one of the few commercially available books that focus on lens manufacturing as opposed to lens design. To be sure, camera lenses are not high precision optics in this context Wink.

Useful references for cementing lenses (both still in print as of Jan 2022):

- Optics Manufacturing, Components and Systems", Christoph Gerhard, CRC Press, 2018 (cementing with modern cements)
- Prism And Lens Making, Second Edition, F. Twyman, CRC Press, 1952, 2017 (detailed info as to how to prepare Canada balsam solutions for cementing, and on balsaming jigs)


PostPosted: Sun Jan 02, 2022 12:04 pm    Post subject: Reply with quote

RokkorDoctor wrote:
Really looking forward to seeing the results, I would be interested in a feasible DIY centering setup myself.

Interested to see how you devise a turntable setup that allows for light transmission; I always assumed any DIY attempt at that would have too much play/runout in the bearings to be of practical use, as the wobble circle of the transmitted light should be quite small (ca. 0.1mm in the example of a 50mm focal length lens with unity magnification of the optical test setup, allowing for a 10 arc minutes decentering tolerance).

As I understand it, decentering tolerances are usually in the order of some angular minutes, down to some angular seconds for high precision optics (source: Christoph Gerhard, 2018, see below) Very interesting read that book by the way, one of the few commercially available books that focus on lens manufacturing as opposed to lens design. To be sure, camera lenses are not high precision optics in this context Wink.

Useful references for cementing lenses (both still in print as of Jan 2022):

- Optics Manufacturing, Components and Systems", Christoph Gerhard, CRC Press, 2018 (cementing with modern cements)
- Prism And Lens Making, Second Edition, F. Twyman, CRC Press, 1952, 2017 (detailed info as to how to prepare Canada balsam solutions for cementing, and on balsaming jigs)


I appreciate the additional resources and interest! I'll have to see about copies of both of those while I'm waiting for the remaining supplies to arrive and try to read up. To be sure, I am a complete newbie to this entire process, and my centering setup idea is devised simply from seeing a several minute video on youtube, so the probability of high precision results are...shall we say, not especially high at this stage. At this point I'm mostly just interested in the fun of trying it and seeing how far I can get. Hopefully any results I do find will allow other DIY'ers with more experience with optics to refine the process, at least.


PostPosted: Sun Jan 02, 2022 4:14 pm    Post subject: Reply with quote

Paragon19 wrote:
I'm probably going to get some actual Canada balsam as well, for comparison, and to practice with separating after adhesion.


Canada balsam is not easy to find these days. I used this German supplier:

https://www.kremer-pigmente.com/en/shop/mediums-binders-glues/62110-canada-balsam.html

This looked pre-filtered to me when I got it, although I still filtered it again and then boiled off the requisite amounts of lighter turpentines to get both a medium and a hard balsam, which I then diluted approximately 1:1 balsam/xylene to saturation. I haven't used it yet but I have a few doublets awaiting re-cementing.

Note: the pure balsam resin contains too much turpentine to use straight as lens cement (it will give bubbles when baked). Boil off turpentines by weight to get the desired hardness (source: F Twyman, "Prism and Lens Making"):

- 7% weight loss for a soft balsam (used for the largest lens elements up to 5 inch)
- 14% weight loss for a medium balsam (usable for most lenses)
- 25.8% weight loss for a hard balsam (for small lenses)

As the turpentines boil off the boiling temperature will go up, but never allow it to go over 200 degrees Centigrade/Celcius, or otherwise you will burn the balsam (will go brown). Even then it is difficult to keep the balsam clear, my medium and hard balsams ended up resp. light yellow / straw coloured.

Canada balsam can sometimes be found in crystallised form, in dark-red/brown lumps, but there all the turpentines will have been lost, so that needs diluting in e.g. xylene (1:1 ratio balsam/xylene saturation) before it can be used.

If you use undiluted balsam, cementing will be done under heating (heated until the balsam reaches the consistency of treacle), and after cementing will require subsequent annealing for several hours in the oven afterwards, the required temperatures depending on the hardness of the balsam used. Again, see Twyman for details:

- soft balsam: edge hardening at 77 degree Celsius for 60 hours, no annealing required
- medium balsam: annealing at 40 degrees Celsius for 2 hours
- hard balsam: annealing at 60 degree Celsius for 2 hours

Diluted balsam (in xylene) can be used at room temperatures, but that will require several days/weeks hardening of the edge, although Twyman does suggest it can be baked at low temperatures (what temperature he unfortunately doesn't specify; I would expect bubbles if too high).

Jon Goodman has a good tutorial for working with diluted balsam on this forum:

http://forum.mflenses.com/re-cementing-doublet-elements-with-canadian-balsam-t34467.html


PostPosted: Sun Jan 02, 2022 10:16 pm    Post subject: Reply with quote

Very useful information, thank you. I have already purchased laboratory grade Canada Balsam in Xylene from Carolina Biological (I live the next city over from them, and even know someone who actually works there), so I'll give that a try first. I'll probably try baking some and record the appropriate temperatures and bake times. Since I'm using these surplus doublets that have no intended use afterward it really doesn't matter how they turn out anyway. I may have to order a few extra though, for the balsam testing.