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Markus, Brian...
Eyepieces usually don't have flat fields. The trouble is, manufacturers don't disclose what the field curvature is so it's pot luck when buying one without trying first.
In a fast small refractor the field curvature is strongly convex towards the eyepiece. The field curvature of a fast 10" f/4 newtonian is exactly the opppsite - concave towards the eyepiece. I did some calculations on this recently and concluded that at the edge of the field of view the disparity between the focal plane at the edge of the field in a 1.25" barrel could be as much as 2mm. for scopes at 1 metre focal length.
So it's really no surprise to me that the series 5000 is a good match in one, but not the other scope.
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