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I don't believe anyone suggested an astigmatic secondary, or an improper secondary mount which would distort the flat. The secondaries are usually very thick compared to their diameter, but this is possible. If the star images from +out to -out of focus are symmetrical ellipses at right angles, then that's astigmatism for sure. If any astigmatism is baked into the primary, maybe you could anneal it out (fat chance) but you would have to re-aluminize or possibly even refigure the primary surface. Another possibility, if you're still reading this, is the eyepiece itself. Have you seen this bad images with more than one of your eyepieces? If you can get some eyeglasses with a very small astigmatic correction, say 1/2 cylinder diopter which is the minimum you'll find, then by rotating the eyeglass lens you may improve the focused star image somewhat. You can buy or borrow an opticians astigmatism only correction lens, i.e. zero sphere diopters, and use that for the test.
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