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That's one very psychedelic shot!
Dust on the secondary would be really out of focus, so it's probably something else.
You should manually look at each sub image. To me it looks like one sub has trailing, probably caused by a bump - You said it only happens with 2/3 stacking algorithms and this is a good hint - some stacking methods can mask this type of problem by rejecting bad data, which is the same way you can get rid of satellites trails too btw.
James
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