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Is that with a refractor using a flattener or reducer/flattener?
I have found with mine that if the spacing is not quite right you get the same sort of effect on the guide camera that you do on the imaging cam (Comets or fishbowl effect) As you generally have the prism out as far from the center of the image as practicable to make sure it does not cast a shadow on the main sensor, whatever effects there are will be really pronounced.
Try tweaking the guider focus in and out a little to see if the star shape changes. While it may put them out of focus a little, if it improves the shape that might indicate the spacing is not quite right.
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