I am looking for information on cementing lenses.
Specifically, I have a 74mm (~3") diameter FK54 and Silica lens set that I want to assemble. The problem is that the optics will be subject to large temperature variations and their thermal coefficients are very different.
Silica: 0.6x10^-6 / C FK54: 14.6x10^-6 / C
Whatever I use must cement the lenses and survive a 30 C temperature variation (from 20 C to -10 C).
To date I have been considering the Norland #65 adhesive. If I must I will use an optical interface gel that doesn't bond the lenses but provides an optical path. The problem with the gels is that you can get bubbles working there way in between the lenses as the temperature varies. Also, since the gel doesn't bond the optics you have to support each separately to ensure that they don't decenter.
My calculations show that it is not possible to cement these lenses together. I calculated the tensile and shear stresses at the full radius of the pair at the 30C temperature extreme and found it to exceed the maximum allowable shear stress of the material (E=20000PSI, Tensile=1500PSI). In fact, the stress is a function of the film thickness of the adhesive. I calculated that the film thickness would have to be at least 0.16mm (0.006") to just meet the tensile stress of the material. I suspect that it would be difficult to join the lenses with this thick a film. I.E. it would be hard to work the bubbles out from between the lenses and still have 0.16mm of adhesive thickness.
I would like to hear from anyone who has practical experience with such problems or anyone who has references to methods of stress evaluation and calculation. In the end I think I'll probably have to make a test sample and try it!
Thanks in advance,
Scott Roberts roberts@dao.nrc.ca