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Thanks Marc.
I think you are right; the trailing at the top corners is due to field rotation as the guide star was inadvertently quite far off.
I tried collimation using CCD Inspector. Managed to get the collimation error under 10 pixels. Any further adjustment kept giving me random variations between 3 and 8 pixels. Even consecutive measurements without touching the collimation screws gave this variation. Effect of seeing?
Thanks,
Vincent.
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