Thank you both for your replies to my query. I’m sorry it took me so long to get back.

I was not able to make a mask out of a circular frame using the Pathfinder. I am too inexperienced, having never used this tool before. I tried ‘Object/Path/Open path’ but didn’t get any further. Thank you anyway Theunis.

The trick from Mike Rankin worked fine. So thank you David, for this and all your other brilliant explanations.

Greetings from Germany,
Nick