http://www.iceinspace.com.au/forum/show ... p?t=178008
I found myself nodding my head at some points where it agreed with my personal experience with some of these eyepiece-telescope combinations, and learning from other points that were made of which I was completely unaware.
In particular, Don Pensack's post #13 includes a web archive link to Pentax eyepiece aberration data no longer published by Pentax
https://web.archive.org/web/20150522023 ... xw/64.html
Examining the diagram, you can see that the sagittal and meridional curves in the Pentax 5mm are perfectly matched as in the 14mm which is why these two eyepieces are so highly regarded.The 10mm, 7mm, 5mm, and 3.5mm all have negative field curvature.
The 14mm, 20mm, 30mm, and 40mm all have positive field curvature.
It depends whether that matches your scope as to whether you see a flat field or not.
Here are the astigmatism/FC curve graphs:
If the two curves (sagittal and meridional) deviate, that indicates astigmatism. The center of the field is at the bottom and the edge is at the top. The scale is in diopters (the eye can usually accommodate 1, but 2 would be seen as curved).
Ideally, the two curves would not deviate and they would be vertical.
The 30mm and 40mm have been out of production a long time, but are coming back soon--perhaps by the end of the year.
My observing partner and I compared views in my 6"f7 newtonian and a TOA150 premium refractor during a side by side comparison of the two instruments that I wrote about in AF early last year.
http://www.astronomyforum.net/astronomy ... ector.html
In that comparison, we made a statement that-
Providing the exact same eyepieces were used in both instruments, we found the views were very similar in both instruments after allowing for the slight (50mm) difference in focal length. The TOA could out resolve the reflector on all subjects but the difference in resolution was very marginal. We had to look long and hard at the finest of details to see a difference. Using eyepieces of same focal length but different brand/types/quality could elicit much greater apparent differences in views, clarity, crispness between the instruments, than the marginal difference that existed between the two instruments when identical eyepieces were used. Both instruments were resolving both the sunlit side and the shadow of the straight ridge in Dopplmayer marked in the attached photos. Each feature, the sunlit side and the shadow, measured off LROC images, was approximately 1km wide. On Saturday night, the Moon was 361000km away from Earth's geocentre, perhaps 352000 km from our topocentric location to the lunar surface suggesting that both instruments were resolving close to 0.7 to 0.6 arc sec, the TOA a little better than the Newtonian.
To elaborate on that statement highlighted in green, the differences between eyepieces in fine lunar details that I referred to through were between mid-range Baader Hyperion 5mm and Pentax XW 5mm eyepieces. The difference in resolving power and clarity of the finest details was very difficult to pick between the two instruments when using the same eyepiece however the difference between the two eyepieces was very obvious with the Pentax giving a much sharper image than the Baader. To be fair, the Pentax costs 2-3 times more than the Baader depending upon which country you are conducting your shopping.
On another occasion, Phil and I had the opportunity to compare a 14mm Pentax with a 14mm Denkmeier in Phils TOA150. We found the views between these two premium eyepieces much closer but the Denkmeier gave a slightly sharper image.