Quote:
Originally Posted by RickS
All true, but the improvement in S/N comes from the additional exposure time not the stacking process itself. If you do a 10 minute exposure and a 90 minute exposure then the improvement in S/N in the longer exposure relative to the short one will also be three times, the same as if you stack nine 10 minute exposures.
Fun stuff, eh?  |
Not entirely true - the signal strength is indeed being increased by the effective integration time, but the noise power decreases as a result of the stack. Consider the image to be composed of signal + a random noise. The next sub is composed of the same signal, but a different realisation of the random noise. Adding the two random signals together gives a new random signal with less power than either of the two original noise components (because the 'noisy' pixels don't occur in the same place), but you get twice the signal. With a bit of simple maths it's easy to show that if the noise is truly random, the signal to noise is now 1.41 times better than either of the originals. Here's a link to one of the awful books I remember from my undergrad last century that describes a related application of the theory...
<http://books.google.com.au/books?id=oRP5fZYjhXMC&pg=PA185&lpg= PA185&dq=stacking+random+noise&sour ce=bl&ots=C9-fMrZslb&sig=VBTxJVg1HUhsRmiZEIwUO_y 7bf8&hl=en&sa=X&ei=MlWBUIeTI5G0iQfS rIC4BA&ved=0CEkQ6AEwBg>
cheers,
Andrew.