Quote:
Originally Posted by Stonius
I'm surprised to hear you say that. I haven't noticed this in any of my scopes, and I notice some of gear is broadly comparable. Sure the XW's have some field curvature, as all eyepieces do, but it's been below the threshold of detection for me. I'm much more bothered by chromatic distortion, atmospheric refraction, or even the barrel distortion in my TV Pan24 (but that's another story - just don't pan with the Pan!).
Can I ask if its worse with certain scopes? Supposedly they're designed with refractors in mind, so I wonder if it becomes more of an issue on very fast dobs (mine is f5).
It's strange though as the XW40-14mm supposedly have positive curvature, and XW10 and above have negative curvature, so you would expect whatever field curvature issues you had to resolve either in the 14 or the 10mm as they have opposite curvature from each other.
Just strange how we can have such different experiences with the same EPs.
Cheers,
Markus
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Hi Markus,
I've owned all of the 1.25" Pentax XW's for about 20 years. The 14mm and 20mm both show field curvature in my 3 Newtonians. 18"/F4.5, 14"/F4.5 and 10"/F5.3. They show slightly less field curvature in the 10"/F5.3 than they do in the F4.5 scopes. While I can see it in the 10"/F5.3 the eyepiece is still very useable and performs very well. I find them still very useable in the F4.5 scopes as well, notwithstanding there is a bit more FC. They actually have less field curvature and astigmatism than the 17mm and 22mm Nagler T4's in all of my scopes when no paracorr is used.
Use a paracorr in any of the scopes and the field curvature completely disappears in the Pentax XW's and the Nagler T4's and they perform superbly. No idea what they do in refractors. I use those as finderscopes
Cheers
John B