Guide stars taken from the edge of the image can be distorted in shape.
I think you'll find the guiding still works fine. Pick a star that's not over saturated.
Make sure your guide camera is focused as best as you can get it.
If your RC is well collimated it may simply be you are picking up the light out of the illuminated corrected circle of the scope.
Guiding software picks the centre anyway so in my experience whilst I prefer a perfectly round star an odd one still works.
Greg.
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