Thank you, @bula. I’ll follow your suggestions and try to reproduce your result.
I have 2 arguments why this 6-panel is actually very hard to get perfect.
1) collimation problems, I noticed that the data was collected during several days. And collimation wasn’t constant over these days. Some panels have good stars in all 4 corners, but some have star shapes that are semi-circles instead of circles. This has a clear effect on the optical distortion pattern in your data, which isn’t constant, and also on the star lokation precision and the number of stars that are found in the corners particularly.
2) Rather small overlap between the frames. The overlap areas are less than 10% of the panels width/height. This is rather small. I suggest to have at least 10% and preferably 15% overlap. This will help a lot in gettting perfect distortion correction and thus perfect registration. More overlap between the panels will also make the success of the mosaic less dependant on not constant collimation over all imaging sessions.
Both are true. Collimation of the scope wasn’t perfect at that time. Focus comes very critical (@ f/2.8). When slightly off, collimation errors show up more pronounced. Especially when the frames don’t benefit from a before and after meridian “dither” (rotation of 180 degrees).
Next time when preparing a mosaic image, I’ll take care in creating large enough overlaps.