Lee --
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... and while the center of the image is fine, it gets progressively worse the closer to the edge (the supplied image being the the top right hand corner of a recent image).
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This makes sense :-) I didn't realize that the image you supplied was a snippet from a full frame. I've had this problem in some of my images.
Field curvature and coma are to be expected in many telescope designs and refractors are not exempt from this, especially in products from the low end of the market (yours and mine). Refractors with a triplet lens design suffer from this problem to a lesser degree. A field flattener would help, but also double-check your focus because even a slight deviation from focus will amplify these off-axis aberrations.