That doesn't look like coma. The elongation of the stars is all in the one direction, unevenly distributed and not radial. Is most likely an unsquarely seated MPCC that makes the sensor not parallel to the image plane or collimation issues. Have a look at your MPCC in the focuser to see whether it looks square, or better still have someone (or yourself) run one of the subs through CCD Inspector - gives lots of good info and cuts out the guesswork
Adam
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