Mark I followed this
http://www.cool.id.au/astronomy/Neb_...d%20Offset.pdf
However it didn't quite work ... Might be me, but it did hive me a good understanding of how the gain and offset really worked, the PDF suggested an increase so
Anyway just trying a 21 gain and 100 offset yuen I will head back into the cols and target a bright star just at 100 just to make sure wells are full.
The interesting thing is wells will be full and without the gain it might have a value of 52000, you can keep exposing but it won't get to 53000. So my understanding is get that value right up to 65 odd thousand to maximize your range, looks like you've got yours bang on. Won't take me long and I will be there.
Might head off the magellanic clouds for some Ha exposures after this 20 mins I'm hoping as subs .