Hi Karsten,
I don't necessarily agree with those comments and assesments.
Quote:
If it was field curvature you would be able to focus the unsharpness out.
But it is not possible to focus it away, so it is not field curvature.
I allways wonder why it is often adressed as "field curvature" in american forums.
It is coma from the mirror and astigmatism from the eyepiece. Both combine
to a large chracteristically formed unsharp blurr.
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I own the 20mm Pentax XW and use it in both a 10"/F5 newtonian and an 18"/F4.5 newtonian. I don't agree with this comment at all. I see the predominant aberration as "field curvature" and 95% of it "can" be focused out and that is the reason it is addressed as such in all the US forums.
This is further confirmed by my own experiences using both my scopes. In that the observed field curvature is worse on the F5 scope, than it is in the F4.5 scope, due to the faster scope having a longer focal length. The observed field curvature is related to the focal length of the scope only, not its focal ratio. Due to a longer focal length scope having a larger "radius of curvature", hence an inherently less curved focal plane.
I said in an earlier reply that there are better choices in your 12"/F5 scope. However, I also indicated that if you "owned" a paracorr that it flattens the field beautifully and you wouldn't find a better eyepiece "if" you intended to combine it with a paracorr.
Cheers,
John B