Markus,
""I don't have ImageJ, whatever that is,""
It's well worth getting ImageJ as a good software tool for many of the things we do (or would like to do) in astronomy. I use it to assist in processing the solar spectroheliograph data.
https://imagej.net/Welcome
The review by Christian Buil may be helpful.
http://www.astrosurf.com/buil/atik_vs_zwo/