If its like that over the whole image, its unlikely to be tilt. Could be collimation, but very hard to diagnose with out of focus stars. I would suggest try again, this time focused and only expose for say 20 - 30 sec. This will / should eliminate guiding issues if there are any and polar misalignment.
seeing you said the elongation was even across the field, this would rule out field rotation from polar misalignment i would have thought.
you can test for tilt with a good repeatable digital focuser and an auto focus routine.
Josh
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