Telescope Reviews: Re: RTV, Glass, and Holder Material

Thanks Preston. What is the best material to adhere two CF pieces together? I've seen references to thick cyano-acrylate. However, I’m still stuck on a couple points detailed below.

Charl, I'm not convinced that wood would be the best, or CF even though FEA has been done on a configuration using that and it seems to be proven.

Since we are creating a sandwich; glass/rtv/holder, the glass/rtv boundary is of concern for deformation of the glass, and maybe that interface should require the adhesive to match glass’s coefficients (or ceramic if that's what actually used), I say that because when forming a sandwich mirror they don’t use dissimilar materials for the front or back plates and ribs, it’s all the same material. So a coefficient mismatch should be avoided. If it’s avoided, then the adhesive layer can be sufficiently thin. It seems thin is the goal as a thick film can allow the mirror to cantilever out of position as the scope is moved in altitude horizon to zenith, but thin films if mismatched can have great stresses and I believe be a mechanism to cause adhesion failure.

However, it seems we’re forced to use something other than glass-matched materials as the adhesive in this application because secondary mirrors don’t come with anything but a flat back. (To look at the problem differently, it’d be nice to have a secondary with a few glass posts shaped like bolt heads that we might hold them captive in some manner that the front surface isn’t affected by expansion/contraction of the captive method used. These posts could be at the ~70% radius, or something else if another pattern is defined as better. Then the holder just clips onto the mirror back. If I worked with glass I’d try this. This may be a bad idea, or not, let me know.)

But, the other boundary, the rtv/holder interface, should have materials matched to rtv, similar to the sandwich mirror requiring similar materials, so the two move together when contracting/expanding I would think. It would seem any holder material drastically different from RTV could run the risk of adhesion failure since once again a thin layer is the goal.

So, what rtv has the expansion coefficient of glass? If there is one, then wood as a holder material may be a good combination, because then all three match. (Though I really like my newly minted “Secondary with glass mounting posts” idea)

Let me know where I’ve gone astray.
Steven <<<goes astray quite often (sometimes it's the journey, not the destination necessarily, that's of interest)

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Mr. Wizard

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