CN FORUMS ARCHIVE Telescope Reviews: Field curvature with 19mm Panoptic ?

The explanation may lie in the fact that the 24mm does not MAGNIFY the blur circle enough to RESOLVE it.

I see this problem in SCTs all the time. At very very low powers, the field curvature of the telescope is at its WORST because you are going to the extreme edge of the mirror. But when using very low powers, the blur circle (the total width of the abberated image) is so low that my eye can only BARELY resolve the unfocused Fresnel pattern. It still appears more or less as a small blur.

But if I plug a 17mm Nagler into my C14, at the EDGE of the field, the stars are so unfocused that they actually resolve as "Unfocused Stars." In other words I can actually SEE (resolve) the Fresnel Patten, just as if I were placing the star at the center of the field and unfocusing maybe 3 wavelenths!!!

Now this eyepiece in my 140mm f/5.7 refractor is sharp as shark teeth at the very edge of the field. It is literally PINPOINT sharpness at 41.5 apparent degrees on either side of center. Only when the point is ON the field stop does in show any abberation in that scope.

If I FOCUS the edge of the field, it will look EXACTLY like I am racking to focus on a star at the center of the field. But of course when the edge focuses, the center defocuses. But again, because of the much higher magnification, the Blur Circle actually becomes resolved.

And I think that is what is happening to you. Using the 24mm, the diameter of the blur is much smaller, so consequently, you simply can't resolve the abberated image well enough to see the Coma present that you see by using the higher power eyepiece. Even though you are using an area closer to the center of the field, fast reflectors are only diffraction limited in a very small area at the center of the field, so there is PLENTY of coma available to see.

So, unless you are using a coma corrector, I would EXPECT you to see some coma using the 19mm in most larger fast scopes using the 19mm. But with the 24, you simply may not be magnifying it enough for it to be glaringly apparent.

But this is me. Like I said, I personally have tried 40 or 50 eyepieces in my Vixen 140, and the only ones that were esentially sharp from field stop to field stop were the Radian, Naglers, Panoptics, and the Meade 8.8 UWA and the new Meade 18mm UWA. All of these eyepieces were pretty much pinpoint performers in the Vixen 140. I have never tried any Pentax, but I have tried old Meade SWA, University Optics, GSO, Meade QX (the 26mm Meade QX 70 was simply the single most aweful eyepiece I have ever used) WO 66 degree AFOV widefield, Plossls of all kinds (including Televue, which suffer from LESS astigmatism far off axis than most other brands but still show Astigmatism, making them my least favorite Televue eyepieces except for on axis planetary viewing, where they are the best thing I have ever used).

So, it is only my guess that the magnification is giving you this result.

But your experience is your experience. If the eyepiece really doesn't suit you and you really believe that this and other 19mm Panoptics have astigmatism, then I suppose the best thing to do is sell it and look for another solution.

But before you do that, you may want to find someone in your area with a long focus Rumak or a Televue NP127 or other telescope with an esentially flat, well corrected field to try it in. My bet is that in a telescope like this, the 19m Pan will be pinpoint to the edge. Now if you do this and find it to be the case, you STILL might decide you don't like the eyepiece. For example, I don't use my 17mm Nagler in my C14 too much because this particular combo results in JUST enough magnification of the blur circle and over JUST enough of the mirror that I find it less pleasing to use than the 22mm or 12mm. So I simply don't use the 17mm T4 as much in the C14 as in my Vixen 140, where it give a GLORIOUSLY sharp 50x, 1.75 degree field. In fact, in that scope, it gets used almost equally to the 31mm Nagler and the 5mm Nagler. All of these eyepieces in the vixen provide exquisite widefield performance.

So, this is just my theory, but it is based on a lot of experience using these different eyepieces in different scopes.

Don't use a standard Mak though. Most of these scopes have some coma off axis. Only a Rumak with aspherized secondary will be able to show how free of abberations the Panoptics and Naglers really are.

Just to kind of tell you what it will do to you though.. I had a Vixen 140 and I sold it. But it spoiled me so stinking bad in terms of showing how incredible wide field viewing COULD be, I lasted about 3 or 4 months, and bought a used one for more than I sold my new one for.... And never regretted it for an instant.

If I ever buy another refractor, it will be a TV NP 127. Once you have used these eyepieces in a scope like these, you won't be able to imagine life without it.

Regards.

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Celestron CGE 1400
Astro-Physics 6" f/8
Celestron EdgeHD C8
Antik AR1/Revenge Plates/Heartless Voodoos

The night sky is my mistress. She seduces me away from all other lovers.