Quote:
Originally Posted by Joshua Bunn
On page 40 of the book you referenced to it has an example of the S/N ratio being the same for stacking via summing and then averaging. Both methods produce the same S/N ratio. But would the summed set have higher pixel values? (greater signal, but also greater noise, as it hasnt been divided by the number of sub frames taken). I know summing is not used because it has some artifact drawbacks in the final image but that s not my point.
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Yes, you do get greater pixel values by summing instead of averaging but the absolute values don't matter when you stretch the data. The amount you can stretch is determined by the S/N not by the size of the numbers. You can only stretch until the noise starts to become objectionable and that happens at the same endpoint whether you start with a summed stack or an averaged one.
Quote:
Originally Posted by Joshua Bunn
The 10 min example you gave, would this be a sum or average, not for the purposes of S/N ratio, but for detecting faint objects. How can you not go deeper with a 10min exposure than 10 one min exposures. I guess the average of 10 one min exposures may show some faint detail.
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Once again, sum or average doesn't matter. How deep you go depends mainly on how many photons you collect in total from the target object. You are trying to collect a lot of photons not to make the pixel values bigger and make the object "brighter" but to reduce shot noise as much as possible. If shot noise is low then you can stretch the image more and see more detail.
I say "mainly" above because there are several other factors such as read noise, dark noise, noise from the skyglow, etc. that come into it.
Cheers,
Rick.