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Is astigmatism normal?
Ok, dumb question on the face of it, but I have to ask. I've seen mention that *off-axis* astigmatism is inherent in newtonians, but it's usually overwhelmed by coma. When one introduces a coma corrector then, is it expected to see off-axis astigmatism in a "normal" set of mirrors that are well-mounted?
I understand that on-axis astigmatism is definitely abnormal, but I'm confused about off-axis.
The reason I ask is that I can see what I believe is off-axis astigmatism. In the attached example I've taken a section of an image showing the same stars from the top left corner of the frame. One was outside focus, one at focus (roughly) and the other inside focus. The in/out show perpendicular elongation and the "at focus" image looks a bit misshapen. On-axis stars look fine, I only see this increasingly towards the edge of the frame.
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