Malcolm, I battled with elongated stars for months, I tried everything to fix it and in the end it came down to a lens in a coma corrector that had a very slight tilt in it....wasn’t seated properly. The tilt was only a fraction of a mm however it was enough to split the stars in my image into a slight spectrum, which ended up looking like elongated stars unless you looked really closely.
My advice - check everything is seated correctly and your lens/filters aren’t playing games with the light path
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