johnpd I don't notice any sizable elongation on the corners.
Huh? I took a look at just the top right corner (and the 533 is not even a large sensor to start with), and already saw this:
Since the center stars are rounder, this much elongation cannot be completely blamed on guiding, but the optics. Could be a tilt, too, since a tilt changes the "backfocus" for different corners of the frame.
On even an APS-C sensor, it would look much worse.
See OPT's backfocus white paper "Camera too close":
https://optcorp.com/blogs/deep-sky-imaging/how-to-set-the-correct-back-focus
BTW ,read my earlier postings about short spikes in guiding. If the duty cycle of the guiding deviations are small compared to the total exposure time, you won't see them on the final image. They do very little harm.
Chen
