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Hi David,
I have a fast scope and OK sensitivity camera, so I find that after 10 to 15 minutes (target dependant) the SNR is enough. I also find the longer you go the more time you lose to unexpected things happening, like planes, satellites etc.
As you go longer in your subs, you will inevitably take less of them, so the statistically based rejection algorithms used in data processing are less effective, bright satellite trails aren't completely removed for example. So this is also a consideration I have when deciding sub length. As a compromise I'll tend to shoot 10 minute subs as I find the SNR is usually pretty good and the greater number allows the sigma rejection to work much better (which also increases the SNR). Really dim targets require greater exposure length mainly to get the S part of the SNR up.
So, for the Rosette nebula, I can easily get away with 5 minute subs for all three filters, though, from memory, there isn't much SII anyway.
Cheers
Stuart
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