This is the same image as the first image in this gallery.
It has had a big stretch applied to highlight the effect of the flat frame.
As can be seen, it has removed the severe vignetting,
but there is still some vignetting to deal with in final processing.
This frame also shows a lot of gradient problems
that are removed in final processing.
It highlights the benefit of stacking,
since the noise can be processed out with the greater signal obtained in stacking.