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Aley Elagin

Split diopter telescope shot

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Hello everyone, need your collective experience to come up with solution to this shot. I need to take a shot through the astronomical telescope and see the moon. It’s an old Zeiss Jena telescope, with no possibility to attach camera directly to it. At the same time i need to see a face of the telescope viewfinder, as we attach something to it. Both image projected from the telescope (i.e. moon) and viewfinder face have to be in focus at the same time. I was thinking to use custom made split diopter lens, with hole in the middle. How do i calculate needed diopter and can simple magnifying glass work for it? Or maybe there is completely another solution available, that i’m missing?

Thank you!

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It's a static detail shot, isn't it? Does anything moving obscure the image in the viewfinder? 

You could shoot the image in the telescope separately with the exact focal length needed to get the right image scale far enough from the eyepiece, and composite it into the shot. 

As to focusing, the image of the moon will come out of the telescope in practically parallel rays, which means focusing the taking lens around infinity. And the focusing distance for a (split) diopter will be 1/D if we need parallel rays after it. A short-focus single-element diopter might unacceptably degrade edge definition and introduce CA though.

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I don’t think a round hole in a diopter filter will work if you want the circular edge of the viewfinder in focus and the image inside the circle to be in focus. That’s too small of a transition for the blurry transition in a split-diopter and I doubt you can stop down enough for a sharper transition, which then risks seeing the cut edge of the glass diopter. My suggestion is to build a fake rear end of a telescope with a slide transparency of the Moon just inside the viewfinder and then backlight it to illuminate the slide. Or do a post composite.

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