These were shot through a 45degree inclined "hot mirror" inside my beam splitter that introduces astigmatism into the optical path
the beam splitter can be seen here and here (note the image of the bubble part way down the page)
I found a better way to collimate the scope yesterday and am happy to see that the "hot spots" in the out of focus dougnuts were corrected.
First shot taken after collimation:
Click to enlarge
(3 x 20 minutes Halpha, 2x2 binned IMG6303, 18" classical cass @f/12.6)