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Generally you are trying to increase the s/n (signal to noise) ratio. The noise comes from many sources, ccd read noise, thermal noise, cosmic ray hits and so on and is random in nature; while the signal, nebulae, stars and so on is constant.
Stacking amplifies the signal but randomises and reduces the noise, if you stacked the same image over and over you would also be amplifying the noise.
You wont see things that are not there, i.e. a 60 minute sub will always be deeper than 6 by 10 minute stacked subs but the signal visible in a stack of 6 by 10 minute subs will always be stronger than 1 ten minute sub; and the image will look smoother and less noisy.
Cheers
Paul
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