I discovered that the lens I purchased (used) had a UV drop-in filter inserted. So I removed that filter and tested again the last night, under a full Moon… On an EQ8 without any guiding, did 18 times 60 second 800ISO shots on F/4 using BackYardNikon.
There are still skewed stars left in the corners, especially to the left lower side. But that might fall within normal DSLR tolerances. I do not understand the assymetry though / left to right.
The only thing that still worries me is the darker circular donut in the middle of the image. It might be related to the large hood that this lens has, a sunshade. I need to test it without the hood (and with a dew band).
The bright star in the middle is Deneb, the red nebulosity at the top is the Pelican.