Tell me if I got this right now:
1. Parallel rays that are not parallel with the optical axis of the mirror are not focused to a single point. That kind of off-axis aberration is referred to as coma.
2. The other type of off-axis aberration associated with parabolic mirrors is that the image plane is not really a plane but a curved surface. The faster the scope the greater the curvature. That's the fast light-cone business, right?
3. Coma corrector like paracorr corrects #1, and top dollar eyepieces look after #2.
Am I even close?
BTW. It would be much easier to convey these things in diagrams. I myself was too lazy to produce any.
PS. do a google search for "light cone"!
