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Mixing subs of same target captured at different locations
Let's say that I have a total of nine hours of M81: three hours from a Bortle 6 site, three hours from a Bortle 5 site, and three hours from a Bortle 4 site. Furthermore, let's say that I use the same scope, camera, exposure and gain. If I were to process these together to make one final image, can I sort-of expect that it would be equivalent to capturing all nine hours at the Bortle 5 site in terms of SNR? Is there a "gotcha" that I don't understand? Clearly, the background sky ADU will be different at each site. Does APP "normalize" this out?
So it will help to combine all this data and get better signal for sure. However, the quality of the data from the sites with more light pollution will affect the end result. So yes, it'll be better and normalised nicely, but there will be more noise and less signal compared to having all 9 hours at the darkest site.