There is lots of noise you don't get removed by averaging, ie the base noise of the sensors. For those you usually (for amateur astrophotography) take four sets

There is lots of noise you don't get removed by averaging, ie the base noise of the sensors. For those you usually (for amateur astrophotography) take four sets of pictures (usually about 20); with the lens removed and cap on the mount, lens mounted with lens cap, lens mounted into telescope but telescope cap on and lastly telescope aimed at target but a thick cloth over the opening. All those compensate for the various ways light may enter the camera (lens mount, lens frontmount, telescope housing, telescope lenses and mirrors) and let you subtract those out after averaging your result.