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Most of that astigmatism is probably caused by your interferometer. I'm using a tiny lens and prism to minimise the displacement between the two beams. My spacing is something like 6mm. How much is on yours?
Anyway you have to make 5 sets of measurements, rotating the mirror 72 degrees between the sets, then average each set, derotate each set and combine for a valid final result. A big undertaking.
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