Thanks Ross,
Here is the same set but using FLATS!
I almost never use flats, I'd put that down to my stubborn belief
that the CCD is usually clean and there are no obvious vignette issues.
Well by lucky accident last night I started a set on NGC 6744 and went
inside to make a mid dawn cawfee.
When I came back out I had almost nil good subs of 6744 but 200 subs
of some very even grey cloud deck.
They showed the dirty CCD very well. Ambient conditions were very similar
to the previous night subs, so I thought I would see if I could use
the cloud sets as Flats.
Hope I have this sequence right but then I did this:
Subtracted master dark from flats.
Made master flat.
Calibrated 29th march NGC5128 lights with 31 March master dark
and 31 march master flat.
Upsampled then stacked calibrated sets and stacked as normal.
Set without flats (29 march) image on left, Set with flats on right.
Master dark Centre, Master Flat right.
Quite amazed that the flats removed all traces of dust and even let me
sharpen a bit more aggressively.
Hmmm, maybe there is something in using flats after all
Steve