Quote:
Originally Posted by ausastronomer
Hi,
As you can see the 3.5mm, 5mm, 7mm and 10mm Pentax XW's have inherent negative field curvature. When used in a newtonian the +ve field curvature of the telescope cancels out with the -ve field curvature of the eyepiece and you get a flat field view.
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A lot of wishful thinking in that statement.
The cancellation would only occur at one single telescope's focal length (about 500mm judging by those graphs); and only for ONE Petzval curve (saggital and meridional curves are quite a bit apart, indicating eyepiece's residual astigmatism).
In my experience, no Pentax eyepiece gives sharper images at any given distance off axis than a corresponding Nagler. Naglers are significantly wider, sharper off axis, smaller (type 5 and 6), work better at fast f-ratios and can completely take care of residual astigmatism of the observer's eye via Dioptrix, if needed (eliminating the need to wear glasses while observing). I can see people liking Pentax eyepieces, but for me they hold no interest whatsoever. They may be a
smidge more neutral in tone, but that is about it.
BTW, Newtonian's field curvature is rather unimportant (unless you use 8" f/4 or shorter as your main scope). At 14" (or 18") at f/4.5 telescope's curvature is negligible (10mm off axis, the curvature would amount to about 30 microns, worst case. At that amount, defocus blur is barely larger than Airy disc, and is completely swamped by coma).
I think a lot of people prefer large eyelens because they use eyepiece as a screen (to look AT, as opposed to THROUGH). I actually find huge eyelenses annoying and rather distracting.