[Wavytone]

Kim,

You're only looking at eye relief and spherical aberration of the exit pupil (the "kidney bean" effect). This is by no means the most important - and you've omitted the most important part - the telescope. Not all eyepieces suit all telescopes - quite the contrary as most have found out the hard way. What you see are the sum of the aberrations of the telescope AND the eyepiece. It is possible for example to have an eyepiece that suits a refractor that is very ill-suited to a fast Newtonian, and vice versa. Similarly an eyepiece that suits a Meade or Celestron SCT probably won't be great in a refractor.

In short:

a) it is essential to know the focal ratio of the telescope you will use this with, and the corresponding depth of field; what works well at f/15 or f/10 probably won't work at f/4.

b) you need to compute - by- raytracing - the field curvature, astigmatism and coma of the telescope,

c) find an eyepiece design that works well with an incoming beam of the same focal ratio as the telescope across the whole of the field of vie, i.e. at the edge of the field stop as well as the centre of the field. Many eyepieces work well at f/10 but are rather poor at f/4.

d) choose an eyepiece that has field curvature that is approximately the same as that of the telescope. If the field curvature is at least of the same sign its promising; if it is of opposite sign the eyepiece + telescope combination will be terrible.

e) find an eyepiece design that has coma and astigmatism opposite to that of the telescope, the net effect being to cancel the contribution from the telescope.

f) eyepiece designers usually don't bother to correct distortion and put that in the too-hard basket.

Beyond that.. if you are really into custom optics it is possible to match the chromatic aberrations of a refractor and eyepiece. This is not done for consumer grade eyepieces, however some Zeiss refractors were designed this way, where it was assumed the scope would always be used with the particular eyepiece (i.e. the eyepiece is not interchangeable). Some military equipment - e.g tank and gunsights - are also designed this way, and many people buy them on the surplus market only to find they are not suited to use with commercial eyepieces.

From memory Conrady provided the formulae for designing a complete Dialyte telescope (fast singlet objective, smaller negative singlet lens halfway down the barrel and a matching two-element eyepiece) that performed rather well for its day, but the eyepiece and telescope only worked as a matched pair and could not be interchanged with anything else.

By way of example it is fairly obvious that Televue and Explore Scientific are designing their eyepieces to suit fast Newtonians around 30-50cm aperture at f/4, and have built-in some degree of negative coma and matching field curvature. There is simply no way that any eyepiece could give pinpoint images across a 40mm focal plane in an f/4 telescope without this. These same eyepieces will perform adequately in an f/10 schmidt cassegrain (Meade or Celestron) because the depth of field is enough to accommodate some of the field curvature, however in a fast refractor (f/7) they perform rather poorly because the field curvature doesn't match and the depth of field is too shallow to cover this.

For the same reason the humble Plossl eyepiece turns out to be an excellent match with Newtonians as it (surprise surprise) also has a curved focal plane that is a close-enough match and negative coma.