[Wavytone]

Alex,

"better corrected" usually means aberration free, but doesn't usually equate to "best" in an eyepiece.

In an optical system the aberrations in the image is the sum of the aberrations from the optical components along the way.

An eyepiece that has a curved field and a modest amount of negative coma will give superbly sharp images across the field in a Newtonian, far better than eypeieces with flat fields and no coma.

The point about the RKE's is that Edmund designed them to match the old Astroscan - which was an f/4 Newtonian, hence why they suit your scope (it's almost an exact match), and they do this WITHOUT the need for coma correctors or field flatteners - which actually would make the image WORSE with the RKE's.

There is no other eyepiece that will give you such a good match with just three elements. There is one downside - a fair amount of pincushion distortion.

If you wanted to use "better corrected" eyepieces, you'd also need a coma corrector and field flattener to eliminate the contributions from the primary mirror; but with so many more pieces of glass in the path the image may be degraded in other ways.

Conversely, while the RKE is OK in an SCT or Mak, many eyepieces can give much wider sharp fields because the light cone is so narrow - f/10 - f/15 - that the contribution from the eyepiece won't matter.

Similarly, for refractors, depending on what sort of scope you have, the colour corection of the eyepiece may either make matters worse, or better, depending on the characteristics of the eypeiece and scope.

Conclusion ? The "best" eyepiece depends on the aberrations of the scope you will use it with.