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Hi,
I own this eyepiece which I use in my 18"/F4.5 and 10"/F5 dobs. I also own a 17mm Nagler T4.
Geoff is spot on in his assesment of the 20mm Pentax XW. It will show some field curvature in your scope. This is its only weakness. The field curvature will be eliminated if you use a paracorr. The 20mm Pentax XW has many strong points, including its excellent sharpness, contrast and light throughput. It gives a very "clean" view because of its cool colour reproduction. It has very good eye relief (20mm) and is very comfortable to use. If you own a paracorr you can't buy a better eyepiece around this focal length. If you don't own a paracorr and don't plan to buy one, the 22mm Vixen LVW, the 21mm Denkmeier and the 22mm and 24mm Televue Panoptics are likely better options for you in 1.25" barrel format.
Cheers,
John B
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