Hi Tom,
Okay, you are starting off with big mosaics already ;-)
First thing we need to know…
Do you remember how you registered the individual panels in Pixinsight?
If you used any form of thin plate splines, chances are, PI has done more damage to your data than good. So we need to know this before anything else.
Explanation being: the thin plate splines are a terrible solution to correct optical distortion, they don’t correct the reference and the other frames are distorted just to fit to that uncorrected reference frame. PI calls it distortion correction, when it actually isn’t…
Real distortion correction corrects all your data (including the reference) to the rectilinear projection it is supposed to have ;-)
So have you used thin plates splines in PI? If you did, it’s possible better of we take another approach.