Just a simple question is there a number I am looking for in image statistics for a good exposure using my CCD camera? I have been using narrowband filters up until now and I usually captured 20-30min sub but now that I am using LRGB I am just not sure when I have collected enough per sub. I tried 3 mins for a Lum maybe I could go 5. I am guessing it involves mean or median but not sure if there is a value I should try to get to and for which one.