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I did think about the Hershelian but it needs to be quiet large and of long focal length not to need a corrector for off-axis observing.

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You're maybe thinking of the way it's often depicted, a normal paraboloid (or even long FL sphere) tilted. To make it properly it should be an off-center paraboloidal section and then it makes into a nice instrument, albeit a flat helps to fold it back to where the viewer is. Gary Fiskhorn made one of these, you can find it in WillBell's The Best of Amateur Telescope Making Journal Volume (or in the magazine if you have it.)

So add one more surface, that's 4 total with the Tolles EP. (I don't know if it would actually work for the Herschelian though!!!)

Best,
Mark

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