These stars look like Saturn. Coma?

Was goofing around with a rented Sigma 35 1.4 A1 a week or so ago and thought everything was looking good until i get them onto the computer.

Can someone explain why these stars look like they have a ring around them? This is taken from the upper right corner, the center is not so bad. If it's coma, does stopping down help?

@ 1.4 15 seconds ISO 3200

Yes and yes. I can't remember if it's specifically coma, but it's one of the spherical aberrations. If you did a wide field with just stars, you'd find that each corner would have the same thing symmetrical around the center. Stopping down fixes it.

If it's not symmetrical around the center then the sensor plane isn't square. You see that sometimes with fast lenses on mount adapters.

Steve.