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If it is astigmatism, the axis of elongation should should change by 90° as you move either side of focus. (Don’t let any astigmatism present in your viewing eye confuse what you are actually viewing either.)
Also, can try rotating the primary in its holder to see if the aberration follows the rotation.
If the primary cannot be rotated, be wary of the mounting ‘system’. There have been many examples where the use of silastic as an adherent has put enough strain onto the primary to cause astigmatism in an otherwise good mirror.
HTH
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