Is off-axis aberration more important for fast Newtonians?
A. There are no great differences on-axis. Conventional wisdom that an ortho is not well suited to a fast Newt or a “steeply converging light cone” is completely wrong, as long as we are talking about inspecting small objects that lie on-axis. They do break down more quickly at the edges in fast newts primarily because coma is added to the eyepiece astigmatism and all conspires to mess up the image, but also the field is narrower than in many eyepieces so this is not as noticeable as one might think. Pretty much the same goes for refractors – f/5 to f/15, the planetary performance is really not all that difference as long as the refractors have good color correction (which is not always the case). The performance (in both cases) on starfields and the like will appear radically different, of course.
