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Hi Rex, the first thing I'd do is rotate the camera in increments of 90 degrees then shoot flats and compare to see if it's sensor/shutter related or something else in the imaging train such as uneven illumination or vignetting. Which camera are you using? Something with a mechanical shutter? Shutter ghosting on flats under 1s duration can be an issue with a QHY9 for instance.
FWIW astigmatism wouldn't cause this. Collimation in a newt with a shifted light cone due to a tilt in the focuser to compensate for an offset secondary will certainly do something like this.
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