I’m not familiar with ASTAP but the file appears to have a WCS and has a record of some lights darks and flats. The keyword CALSTAT is also suggestive ie DFBS – dark, flat, bias, stack?
When you display it does it look like the original frames but with less noise? If so I would assume its your stacked image. You could check that by using a different stacking method (SUM? ADD? Total?) and see if the sky count jumps up. Has any vignetting present diminished? Those would be big hints.