Hi Josh,
Do you use CCDInspector? It might tell you something your tests up to now are not seeing and could be worth a go. There is a free trial if you don't yet own this program. If you shoot a nice star field it will give you field curvature info as well as optical alignment. Another function will give you a plot of star aspect ratio over the whole field. It would be easy to compare shots in different portions of the sky. This "might" give you something more to work with.
Peter
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