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Hi Tony,
Yes you are reading this correctly. As can be seen from the diagrams the 3.5mm, 5mm, 7mm and 10mm Pentax XW's all have -ve field curvature. When combined with a fast newtonian which has inherent +ve field curvature the two cancel out and present a lovely flat field view. The 14mm and 20mm XW's have +ve field curvature, which when combined with a newtonian which also has +ve field curvature, compounds and gives a fairly soft view at the edge of field. The 30mm and 40mm XW's have a reduced amount of +ve field curvature and give reasonably good views in a newtonian. A paracorr whilst designed to correct for coma, has inherent -ve field curvature which cancels out a lot of the +ve field curvature of a newtonian, consequently it flattens the field noticeably when used with the longer focal length Pentax XW's (14mm and longer).
Cheers,
John B
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