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Hi Lars,
The bottom half of the stars look pretty round and good with a little tail on some (possibly guide errors?).
But the top half is showing bad coma so that is tilt.
Your sensor is not square to the lens. The lens is quite heavy so perhaps its putting pressure on your adapter.
Regardless the solution is to pack out the offending side top or bottom of the sensor depending on whether your image is reversed from the physical sensor.
I do a test to determine how the image corresponds to the sensor.
Take a 10 second exposure and use a dimmed down torch (with a cloth or something over it) and expose only the top half of the lens.
Look at the resulting image, is it bright on the top half or the bottom half of the image.
So now you know which side of the sensor - top or bottom, matches the image.
Now get some thin packing material - spark plug gauges or thin sheets of metal. I got some thin brass sheets from a steel company. They are thin like paper.
Cut a small strip and pack it under the middle of the side of the camera that corresponded with the top of the image you posted.
Refocus and take a test image and see if the seagull looking stars are now round or not.
You may need to repeat this several times until you get it right.
One tip, if the stars are getting worse after packing then its either the wrong side or too much packing. Usually the wrong side of the sensor is being packed. It should improve not get worse.
I have used this lens and got round stars to the corners of a 16803 sensor which is larger but a bit forgiving compared to CMOS sensors with their small pixels that show up defects rather easily.
Greg.
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