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Old Thursday 14th December 2006, 14:00   #10
henry link
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Join Date: Feb 2004
Location: north carolina
Posts: 3,521

Thanks for you additions. I'm sorry I didn't have an MC II available since that is the only one that can be purchased now. I was surprised to read about the improvements in off-axis performance, which I think is already pretty good in the MC. Your mention of using a bar target at the edge made me realize that I had been a bit too casual in evaluating edge performance. I used the USAF 1951 target, but was lazy about recording measurements, so I think my descriptions are too vague. I compared the eyepieces again, this time recording edge measurements and found some differences from my original descriptions worth reporting, so here I hope is a better description of edge performance in the AP scope.

Firstly, all the eyepieces have a bit more astigmatism than I reported. At some focal lengths it is significant. At the lowest magnification the Swaro still looks the best at the edge of its 40 degree field. There is only a loss of about 1 element on the chart which is only about a 12% drop in visible detail compared to the center. The Nikon looses 2 elements which is still quite good. The loss at the edge of the Baader's 49 degree is naturally worse, a loss of 4 elements, about 50-60%, mostly from field curvature. The Zeiss is considerably worse with a loss of 8 elements, about half from field curvature, the rest from astigmatism. What that means is that if 4 arc second details are visible at the center, then the Baader will show about 6 arc second details at the edge while the Zeiss will show details no smaller than about 12 arc seconds. Of course no one looks at details at the edge, but this is the only way i know to quantify the differences. At the edge of a 40 degree field circle (same as the total field width of the Swaro and Nikon) the Baader improves to nearly equal the Swaro with a loss of only one element, while the Zeiss improves to a loss of 3 elements.

At the highest magnifications all the eyepieces have much wider apparent fields so these results cannot be directly compared to the low magnification results. At the highest magnification the Zeiss is a bit better than the Baader, showing a loss of 5 elements at the edge of a 70 degree field compared to a loss of 7 elements at the edge of a 72 degree field, in both cases astigmatism accounts for about a 2 element loss and the rest is field curvature. The Swaro loses about 4 elements at 68 degrees and the Nikon MC about about 6 elements at 58 degrees. An MC II should do about 2 elements better. In the Swaro and Nikon astigmatism dominates, accounting for 3 elements in the Swaro and about 5 elements in the Nikon MC. To see how these results compare to a premium quality fixed magnification eyepiece I tested a Pentax 14mm XW. It showed a loss of about 5 elements at the edge of a 70 degree field, almost all from field curvature, quite similar to the Zeiss.

I should caution that these results were obtained in a well corrected f/6.6 telescope. Off-axis performance in other telescopes will not be quite the same. It should be better in high focal ratio scopes and probably, but not certainly, worse in faster scopes. Another unpredictable variable is vignetting. The AP scope is designed to completely avoid vignetting with even the widest field, long focal length eyepieces. It's surprising how many astronomical refractors show some partial vignetting from the focusing tube or some badly placed tube baffle and spotting scopes are often worse because of undersized erecting prisms. Most of the other characteristics I tested for should still be true for other scopes, but off-axis performance (except distortion) is less predictable.


Last edited by henry link : Thursday 14th December 2006 at 15:27.
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