Updated technical material on roof prisms and phase coatings (1 Viewer)

[Hermann]

Not sure. Brushed under a huge rug that I am not looking under is the following: how well does coating material X adhere to glass type Y? Will the difference stress/strain profiles vs temperate lead to potential reliability problems? I guessing that in some cases, yes. I know from hard drive recording head fabrication done on wafer and various deposition methods that sometimes you have to be take extra steps because process # 18, say, might involve temperatures that adversely affect something you deposited in step #16. Seems feasible to me that optical multilayer depositions might bring up some scenarios like that. So my hunch is that both glass and material mechanical properties may require some extra considerations that simple optical calculation with the TMM cannot address.

Interesting. When Zeiss made the "Serie 25" of 10x50 porros in 1981, I was disappointed that they didn't use modern coatings but the simple coatings of the original design dating back to 1957. I asked them why they didn't use more modern coatings, and was told that multilayer coatings couldn't be used with the glass types they had used in the 10x50. The original 10x50 was an unusual design, as Zeiss used what nowadays people might call an early version of "ED glass" to keep CA down.

Interestingly the 8x50 BA porro, made from 1960-1969, had more efficient coatings; the difference in brightness and contrast is visible. The 8x50 didn't have any "unusual" glass types, at least Zeiss didn't mention them.

Hermann

 

[Tringa45]

Many thanks for your answers.

I should add a point for tenex: wanting to add a layer for anti-scratch etc. is another reason why a single layer AR coating is less than ideal. Harder to "tune away" the problems caused by the durability coating if the AR only has one layer to tweak. Get me many layers and I can accommodate anything.

I see that MgF2 only has a hardness of 6 on the Mohs scale, so is probably much more vulnerable than modern protective coatings.

Plopping down a metal is a helluva lot simpler than fabricating a state of the art dielectric mirror. I also think that with astronomy, especially if you are wielding some monster Newtonian reflector, you can afford to lose reflectivity in a silvered prism near the eyepiece because you've bought so much light-gathering capability via the objective mirror's gargantuan aperture (relative to eyes or bino objectives).

I'm only a casual stargazer (with birding optics) but believe that most primary and secondary mirrors are metal coated.
For lunar and planetary observing with refractors though, there seems to be a recent preference for enhanced metal coated erecting mirrors or erecting prisms with TIR as these are perceived to show contrast better than dielectric mirrors.

John