Recently I spent a few weeks observing with an 8" f/4 Newt under northern skies. Just thought I'd briefly share share my experiences with eyepieces for DSO observing in this very demanding (i.e. fast) scope.
10mm Pentax XW was suited perfectly to the scope. Amazingly sharp stars edge to edge. (NB. I don't make such a claim lightly. I really mean it!). Excellent on all but the largest DSOs that need lower powers. All the good things you've read about the performance of the 10mm XW in f/5 Dobs (and slower scopes) are equally true at f/4. Best all-rounder DSO EP in the 8" f/4.
8.5mm Pentax XF was virtually as good as the 10mm XW with just the slightest bit of softening (field curvature) towards the edge of field. I have to emphasise this FC is very slight indeed (nothing like the 14mm XW even at f/6 for example), and normally goes unnoticed. Brilliant contrast. This EP was the best one for observing galaxies and planetary nebulae in the f/4 Newt, revealing slightly more detail than the 10mm XW.
19mm Panoptic showed some astigmatism (or Newt's coma?) and softening near edge of field and was difficult to use with stars flaring when eye placement is not just right. Newbies found it difficult if not impossible to focus the scope and get a good view through the 19mm Pano. This improved though with practice/coaching. Main use of this EP was to inspect dust lanes of Andromeda and the Veil (with neb filter). I'll probably leave it at home next time.
26mm Meade 5000 modified Plossl (with 60-degree FOV) was easier to use than the Pano but a bit softer and at least as seagull (or coma?) infested near the edge of field. Still a very usable finder (with a FOV near the max possible with 1.25" focuser) and was nice on large objects like Andromeda and the Veil. Less than perfect edge performance a bit annoying on rich star fields/objects, e.g., double cluster. Exit pupil is about as large as I'd want to go at f/4. Barlowed to about 14.4mm effective focal length in the TV 1.8x, edge aberration are minimised, and the EP becomes a very good low-mid-power DSO EP.
14mm Pentax XW stayed at home because I already found the field curvature of the combination with f/4 Newt unacceptably strong. The outer 20 or so degrees (out of 70) are strongly blurred when centre is in focus.
To reiterate my main conclusions
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Pentax XW 10mm and XF 8.5mm are practically perfect DSO EPs for an f/4 Newt. These two impressed beyond all expectations.
)... have fun?
I posted briefly about my northern sky adventures 
