Comet Processing Disaster!

This happens if the comet is moving too „slowly“ with regards to the background so pixel rejection can‘t get rid of all the stars. I‘ld try lowering the rejection threshold and, if that doesn‘t work, to generate 3 stacks, each including only every 3rd sub and then stacking the three resulting imaes. That said, I still have comet data where that didn‘t help me, too. You can also try and use a star removal process on each sub.