Quote:
Originally Posted by Merlin66
Hmmm
Just a question...
You talk about average combine and median combine....
Surely the need is to sum the subs - to lift the total signal...a higher total signal would reduce the shot SNR??
BTW in spectroscopy a spectrum with a SNR of around 10 is taken as the minimum for "useable data" (close to limiting magnitude), SNR>50 is good and SNR>100 is almost mandatory for ProAm contributions.....
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thanks Ken for the SNR info - I guess in determining limiting magnitude all you need to say is "it is there" for which SNR=3 may be enough for a reasonable level of confidence - all seems a bit arbitrary though. As far as I can see, apart from a scaling factor, there is no SNR difference between sum and average if the stacking result is floating point - with average, the noise is scaled as well as the signal so the SNR is the same as for summation. In practice any sensible average would be implemented by a summation followed by a scaling. Median stacking works very well if there are enough subs, but I do not yet understand the relationship to summation - there is a summary in the Pixinsight documentation.