I would like to purchase 2 Newtonian or Schmidt-Newtonian OTAs and build a Dobsonian mount, I was thinking of using mirror diagonals at the focusers so that I would be able to rotate the OTAs to achieve correct interpupillary distance. However, I have seen several catalogs that state that mirror diagonals will not work for Newtonian type scopes. Why is that? Is it simply because they extend the distance from primary mirror to the eyepiece? If so, if I repositioned the primary mirror to correct for the distance would that allow for proper focus?
Yes, that's the problem.
With these designs, however, you also have the problems of getting the tubes parallel, the right distance apart, and having to stand with your back to your target to use them. All possible, just more difficult than using something like double refractors or SCTs (those have the problems of getting the right height on the mount for comfortable observing, and not being able to reach the eyepieces at all in some positions).
With the two designs you mention, the length of the tubes (distance between eyepiece and primary) make the problems worse.
JMI's "reverse binocular" telescopes work quite well, but the interocular design is a bit more sophisticated than what you're considering.
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