Mark,
Mike hit the nail on the head, its all about compromises. Unfortunately eyepieces of different focal lengths in the same series are not always created equal due to physics; the 14mm and 20mm Pentax XW's are no exception. They are not quite as good IMO as the 5mm,7mm and 10mm Pentax XW's which have no field curvature whatsoever in an F5 scope and are oustanding eyepieces in almost every respect. In terms of the field curvature we are talking about a minor aberration when compared to two other premium eyepieces. You can notice it if you look for it and AB it with the Meade 14mm UWA or the 13mm Nagler but when using the eyepiece in isolation it doesn't really affect the view IMO. In terms of cheaper eyepieces the Pentax walks all over them. ie we are not talking about off axis slits like you get in cheaper eyepieces, the star images remain circular at the EOF but visualise tham as being 2% to 5% off perfect focus. Moving the focuser an absolute fraction brings the EOF into focus and takes the center out by the same amount. The field curvature IMO is an artifact of preserving the 20mm of eye-relief in all focal lengths of the series and this is the compromise. I cannot use the Meade or Nagler without my glasses on so my own eyes cause very slight off axis astigmatism in these eyepieces when I use them without glasses on, so we are almost back to square 1. My own eyes off axis astigmatism gets worse as the eyepiece focal length increases due to the increasing exit pupil size. I know however that this aberration is my own fault and not the eyepieces, but there are a lot of eyeglass wearers in the astronomical community and a lot of them suffer the same issues. Its not too bad for me with eyepieces in the 10mm to 20mm range but once the eyepiece focal length gets over 20mm and the exit pupil over 4mm things start looking pretty ugly
. In terms of on axis sharpness and light transmission the 14mm Pentax is a little better than the Meade and also the 13mm Nagler T6, but the Nagler has slightly better contrast IMO. I don't wish to give too much away at this stage as Mike, Louie and I still have more testing to do. We will probably do more this coming new moon and finish the testing next 1st quarter. Mike will then write up the review and post the results. Ultimately all 3 eyepieces are superb and considering the Meade is currently available for under $AUS300 on the used market compared to $AUS460 for the Nagler and $AUS480 for the Pentax, the Meade is clearly the best value and IMO possibly the best value for money eyepiece available on the used market at present, in conjunction with its little brother the Series 4000 8.8mm UWA.
Just as an aside, I regularly use the 14mm Pentax barlowed in the 2.5X TV Powermate for lunar/planetary/double star observation. It gives 220X in my 10"/F5 scope and the powermate eliminates any trace of the field curvature, consequently its a superb combination for medium/high power observation whilst retaining 20mm of eye-relief. I actually prefer it to a 6mm Radian.
Can I ask why you are selling the 14mm Meade UWA, because other than its physical size its a superb eyepiece IMO? Also what are those charts you have attached re the Pentax eyepieces, how can I get a larger readable version of them, curious to have a look?
CS-John B