I often get a similar effect (green images) with the PD when I am doing an extreme stretch. I put it down to a cheap camera! I reduce it by stretching the luminance only and then trim it using “curves” on the individual colours. However, the GIMP light pollution routine you posted does a pretty good job. What do you think of this? I simply used repair or clone on the main galaxy despeckled layer before setting its mode to subtract.
Process used in detail:
- Duplicate image
- Despeckle top layer – set radius to max, black to 0 white to max uncheck recursive and adaptive.
- Despeckle layer now has blob over M101. Get rid of it using clone or repair
- Set mode of despeckle layer to subtract
- Merge down