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Your collimation seems OK. You defocused the star little too much. When that happens, you will start viewing mechanical alignment. Proper star collimation is done at high mag with very few rings showing. If you see the spider vanes diffraction then Mechanical alignment will interfere with the reading. See attachment. I rotated one of the defocused photos by 180 degrees to undo what happens when your cross the focal plane. As you can see, all photos line up. You have a fast scope with a large secondary mirror, hence, the secondary mirror offset will be more prominent on both sides of focus in opposite directions.
However, I do see some astigmatism as you have already noted.
Jason
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