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Hi Evan
Before you invest too much money, it might be worth testing to see if you can achieve focus with the DSLR.
From what I understand (I have never owned/used a Newtonian), the prime focus image from the main mirror comes to a focus only a relatively short distance outside the main tube, where the focuser is positioned. This distance is usually sufficient for an eyepiece to intercept and magnify, but I have heard that some cameras will not come to focus on Newt’s. Basically, the focuser cannot be racked in far enough, so the camera ends up hitting the tube before the image comes to focus. This is termed “lack of back focus”.
If you were to make the diagonal (secondary mirror) larger to project the prime focus image further outside the tube, you introduce undesirable effects due to a larger secondary obstruction.
If you move the main (primary) mirror further up the main tube (i.e. towards the diagonal mirror) then this should cause the prime focus image to come to focus further outside the tube, but you may loose some of the light from the outer edges of the main mirror.
Cheers
Dennis
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