> This is why I take 500+ images and process them. If you do 500 exposures you get 500x read noise of the sensor mixed with signal. Modern CCD and CMOS sensor

> This is why I take 500+ images and process them. If you do 500 exposures you get 500x read noise of the sensor mixed with signal. Modern CCD and CMOS sensor have low read noise (compared to thermal and shot noise) but not zero. For scientific observations of faint objects long exposure times on order of hours are necessary, for amateur astrophotography you can just go with multiple exposures.