Hi Mark.
To answer your last question, your flat indicates that your collimation may be slightly off. one possible cause for the elongation.
As you say, it could be a number of things, if it is til, is is in the upper left and lower right also, but just more pronounced in the lower left.
Try rotating the camera if you can and see what happens to the star shapes relative to the chip. If they follow the chip, it could be tilt. If they don't follow the chip, it could be collimation. Try rotating the camera 180 deg and see if the flat frame unevenness swaps to the other side.
regards
Joshua
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