Quote:
Originally Posted by Fox
There is one area where the Delos is likely to significantly differ in comparison to the Pentax XW line, and that is the field curvature issues encountered with the Pentax XW 14mm and 20mm eyepieces. I found the 14mm Pentax XW field curvature to be too severe for my liking in the f/5 Genesis.
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Hi Fox,
The noticeable field curvature of the 14mm and 20mm Pentax XW's is user and telescope dependant. Some users have a greater ability to accomodate and adjust for field curvature than others. Further, the observed field curvature is telescope dependant. Your F5 TV Genesis has a very short radius of curvature of the main objective and will "dramatise" the observed field curvature, notwithstanding that it is a petzval design scope with an inherently flat field. I spent quite a bit of time recently with both the 14mm and 20mm Pentax XW's in my 10"/F5.3 SDM and specifically looked for and compared the field curvature and off axis astigmatism of these eyepieces with several of my TV Naglers. It's worth noting that the observed field curvature of the eyepiece is dependant on the radius of curvature of the primary objective in combination with the eyepiece. It has nothing to do with the F-ratio of the telescope. That affects coma and off axis astigmatism. Whilst your TV Genesis has a similar F-ratio to my scope my scope has a "much" larger radius of curvature of the primary / objective. I could certainly detect some very minor field curvature in both the 14mm and 20mm XW's in the 10"/F5.3. When I tight focused in the center of the FOV on rich star fields in Scorpius (M6 and NGC 6231 regions) I could tell the EOF was just off best focus. This was minimal and not intrusive to the view, to my eye. Other people may have less focus adjustment in their eyes than I do. If I then tight focused stars 50% of the way to the EOF, the stars both in the centre of the FOV and at the EOF appeared tight focused, even though I could still tighten them a touch with the fine focus wheel on the Feathertouch.
What I can tell you is that the EOF performance of the 14mm and 20mm Pentax XW's in this telescope was clearly superior to the EOF performance of both my 17mm and 22mm T4 Naglers. It wasn't even close. When combined with a paracorr and the paracorr properly adjusted for each eyepiece, they all performed superbly in this telescope with beautiful flat fields right to the EOF. In larger newtonians the 14mm and 20mm Pentax XW's get better and better, due to the longer focal length and radius of curvature of these scopes. In a telescope like a 20" or 25"/F5 these eyepieces perform beautifully, particularly when combined with a paracorr. They perform very well in both my 14"/F4.5 SDM and in my 18"/F4.5 Obsession. To the extent that I prefer the 14mm Pentax XW + Paracorr to the 13mm ETHOS + Paracorr.
I have to be honest I think the field curvature issues of these two eyepieces is way overplayed on most internet forums. Sure they struggle to perform in short focal length refractors due to the tight radius of curvature of the objective of these telescopes, but they do perform very well in telescopes having a longer focal length. When combined with a paracorr they perform superbly in just about all fast newtonians. You seriously can't just make a blanket statement that they have field curvature issues based on your experience in a small telescope with a very short radius of curvature. Funny how you never read about the off axis performance of the 17mm and 22mm Naglers, particularly when they are clearly outperformed in most decent sized visual telescopes by the 14mm and 20mm Pentax XW's.
The best advice I can give anyone about these two eyepieces is to try before you buy if at all possible and do not jump to any conclusions based on one persons performance in a telescope that is different to your own
Cheers
John B