Don Pensack

It was speculated the EOFB was due to its angular magnification distortion, except the Docter/Noblex 12.5mm has no EOFB and has strong AMD as well. Baffles? Lens polish at the edges of the lens? Field stop too large? Unknown cause. But I lent the eyepiece to five other observers, and they all saw the same thing. If your circumstances don't allow it to be seen, keep it, because though tests show it's edge is only average at f/4, its other problem, FC, is very minor, and it was quite sharp at f/5.75.

I find the APM 12.5 to have noticeable and objectionable edge of field brightening. That's too bad, since otherwise it's fine. The 12.5mm Morpheus is a great eyepiece--sharp, great contrast, easy to use. It's also lighter than the APM. It's an easy choice, IMO.

The center and 50% points are very good. It's only at the edge the eyepiece image quality suffers. As exemplified by the Moon photos and Ernest's measurements. However, the figures at f/10 are very reminiscent of many inexpensive eyepieces. It's only by f/4 (and likely f/5) that the eyepiece really falls down in edge quality, and then, likely outside the center 50% of the field.

In order to maintain parfocality, the upper and lower sections must move apart but one moves up while the other moves down. That's why they look like mushrooms at the shortest focal lengths. The Nagler Zoom has 5 elements in a 3 segment upper and 2 segment lower. Al Nagler told me once that he could make a 15-30 zoom with an 82° field in the same manner, but it would be too expensive to sell, too heavy to use, and very fragile.

From Ernest Maratovich's test of this zoom: SVBony Zoom 3-8 8(8.1) 8.1 58(57,3) 56.9 2 6 15 diffr. 5 13 Ast. +6% zoom.. 7 7 57.3 56.5 3.3 5 20 FC,Ast. zoom.. 6 6 57.3 58.4 4 6 15 diffr. 5 11 FC,Ast. +14% zoom.. 5(5.2) 5.2 59.6(57.3) 57.3 4.5 7 20 FC,Ast. +14% zoom.. 4(4.4) 4.6 65.9(59.9) 59.6 6 7 25 diffr. 6 14 Ast. +15% zoom.. 3(3.5) 3.6 68.8(58.9) 59.8 8 8 45 diffr. 8 18 Ast. +15 The first column shows the focal lengths at the click stops (actual measured focal lengths in parentheses). The next column is the field stop at each click stop. No mystery here, because focal length = field stop at 57.3° apparent field, and this is very close. The next column is calculated field stops based on stated (and actual) focal lengths with no distortion. You can use these figures in Astronomy tools to get an accurate answer, or in TF = AF/M. The next column is the measured apparent field (what we actually see). Roughly 57-60°. Then 3 columns of spot size at f/4 (center/mid/edge) and 3 columns of spot size at f/10 (center/mid/edge). 5 is considered by most books to be perfect. 10 is fine as long as the spot stays round. 15 is OK and like many eyepieces at the edge. >20 is so-so. The last column is the nature of the aberrations at that focal length, listed in importance from most to least, followed by a distortion %, if measured. Conclusions: FC dominates from 5-7mm settings. Astigmatism is present from one end to the other. Distortion is fairly high for the narrow fields. f/10 performance is better than f/4, but not remarkably so except from the 3mm to 4mm settings. It has much better performance than most inexpensive eyepieces except at the 3mm setting.

One reason I can think of is to reduce the amount of visible field curvature. | + ) = ) You might be able to focus half way to the edge and accommodate the entire field. | + | = | This is ideal, and reflectors of 1200+mm focal length are pretty flat, so this will work. ) + ) = | And this works if the eyepiece and scope have nearly identical curvatures, ) + ( = bad field curvature, and this can happen with a mismatch of eyepiece FC and scope FC. Since we don't know (the manufacturers don't tell us) whether the FC in an eyepiece is positive or negative, the lowest risk is a flat field eyepiece. But, alas, to the owner of a flat field scope, it doesn't matter whether the FC in the eyepiece is positive or negative--it'll be curved. But, like the person with a flat eyepiece and curved focal plane scope, it might be possible to accommodate the curve, so the degree of FC is important as well. I don't really under stand how short f/ratio refractors of 50-80mm don't see serious FC with nearly all eyepieces, though. My 12.5" has a radius of curvature of ~1600mm. An 80mm f/6 refractor has a ROC of 160mm!! How any eyepiece wider than an ortho functions in such a scope is a mystery.

One note: please go down the page to the next time the Buyer's guide was posted, as it contains some new entries.

These are not new in the world. Most resellers sell them as "Premium Flat Field" eyepieces. Look up reviews on the Astrotech PF eyepieces to see some comments about them. They are also sold by Artesky, Astromania, Astrotech, Auriga, Lacerta, Omegon, Sky Rover, Tecnosky, and Telescope Service. FLO has a good price.

Argh! Another new eyepiece! These are the same eyepieces sold by many companies as "Premium Flat Field" eyepieces. FLO is selling them at a good price.

It is when curvatures match that you see a flat field. When curvatures are opposite, the edge is far out of focus when the center is focused. FC is always more of a problem with larger field stops, too, so FC will be less visible at the 3.5mm end than at the 8.1mm end.

https://astro-talks.ru/forum/viewtopic.php?f=32&t=1483#p101290 Yup. 3.5-8.1mm Review: https://astro-talks.ru/forum/viewtopic.php?f=32&t=4585&p=94406#p94406

And yet, people heat their secondary mirror or corrector plates or eyepieces to prevent dewing. A very slight change in the optical image is preferable to the problem of opaque optics.

The Altair Astro version is heavier than 331g. In an early thread about the green eyepieces, someone quoted the actual weight, but, alas, I can't remember what it was. The APM, with aluminum lower barrel, is 331g.

I sold APM eyepieces for years. On my gram scale the 24mm UFF was 331g without eyepiece caps.

I have just posted the new 2024 Eyepieces Buyer's Guide here: https://www.cloudynights.com/topic/919099-2024-eyepiece-buyers-guide/?p=13395818