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Originally Posted by bytor666
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There are better ways. But first you will ned to understand a little about the aberrations significant in most eyepieces. FWIW it is assumed here that the eyepiece you are considering gives a sharp image on-axis, i.e. it is achromatic, free of spherical aberration and free of wedge, and has been reasonably well assembled. Anything that shows these problems on-axis belongs in the bin.
1. Axial spherical aberration at the exit pupil.
This causes the 'kidney bean' effect noticeable when the exit pupil is close to the same size as your eye pupil - quite noticeable on many low power eyepieces, it is often very annoying with reflectors with a central obstruction, less so in refractors. On short focal length eyepieces ASA is not a concern.
2. Field curvature - if it is a poor match to the field curvature of your scope, you'll always have part of the field focussed and another part unfocussed. Definitely a big issue, what works well on one scope may be a poor choice for another type of scope.
3. Monochromatic aberrations - distortion, coma and astigmatism off-axis. Distortion is only a nuisance. Coma and astigmatism are worse, producing winged shaped that will never focus. The net aberration you will see is the sum of the contribution from the eyepiece and the telescope, so like field curvature, an eyepiece that works well with Newtonians may not be such a good choice for others.
4. Lateral chromatic aberration (LCA) off-axis. Basically, a star is sharp on-axis but near the edge of the field it is smeared radially into a rainbow.
All eyepieces have all of the above to a greater or lesser extent. Worse, when you have a combination of field curvature, LCA coma or astigmatism, you will see some fairly disappointing results for stars near the edge of the field of view.
How to evaluate: Forget about testing on stars, its too awkward and too subjective, and stars are generally too dim to see what's really going on when they are near the edge of the field - you need a brighter test source.
Ideally you need an optical bench with the measuring gear to look into the eyepiece and measure what is seen, but I know none of you have that.
Next best is to use a distant bright light, or an artificial star consisting of a white LED or pea-torch bulb at about 1km range. Even better, use it's reflection off a ball-bearing.