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Supplemental to this answer to Did Edmund Scientific 4¼ inch Newtonians have 90° prisms as secondary mirrors in the 1970's? I was poking around a bit further and ran across this "1960's Edmund Scientific 'Space Conqueror', 3" reflector" with a prism diagonal at http://www.retrotechnology.com/glass/edmund.html

Question: Does anybody recognize what kind of prism this is? It doesn't look like a simple right-angle prism, but possibly something that has additional reflections and does some additional flipping or rotation.

Is there any chance that it isn't positioned correctly, it doesn't look like it is centered along the eyepiece holder's axis, but the image has a strong perspective component and we can't see the business end of the prism.

"Edmund Prism"

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    $\begingroup$ It looks like a pentaprism to me, although it could just be that the housing is a funny shape. I'd say it's misaligned as well, needing to be turned about 45degrees clockwise (looking from the eyepiece tube side. $\endgroup$
    – Dr Chuck
    Jul 19, 2019 at 10:13

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My guess is that it's not a prism (or at least doesn't function as one optically). It looks like it is a right triangle in one cross section. The diagonal face would be silvered to act as a star diagonal. If aligned correctly, the black plate it's glued to that attaches to the support arm would be perpendicular to the tube axis.

It looks like it's rough beveled on other sides, maybe to get an ellipsoidal cross section?

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  • $\begingroup$ I'd agree that it looks more like a simple front-surface right-angle mirror, with beveling to make its profile more of an octagon. The black plate should be facing the outside aperture of the tube. The biggest clue to this (orientation) is that the support post is directly between the eyepiece cutout and the outer aperture of the tube. Good luck with fixing up this old homemade newt! $\endgroup$
    – nflemming2004
    Jul 20, 2019 at 12:21

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