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It might be worth noting that the factor of 10 in the equation isn't necessarily fixed - it represents a trade-off.
The idea is to choose a factor so that sky noise dominates the read noise.
For narrowband, I'd often use a factor less than 10, as little as 2-3 to keep sub lengths shorter.
I think that using factors less than 10 is still valid, it just means that the sky noise doesn't "overpower" the read noise as much.
Using a factor of 10 is good because the total noise is only increased by 5% by the read noise contribution.
The same calcs show that using a factor of 5 would give a 10% increase, x2.5 gives 20% and so on.
My thinking is that using smaller factors can be compensated for by adding extra subs (to achieve the same integrated SNR).
Hence, if you need an ideal 100 total minutes when using long exposure lengths, the following shows how many minutes are required using each factor:
x10 -> Needs 110 mins in total
x5 -> Needs 120 mins
x2.5 -> Needs 140 mins
I've found that sometimes it's better to aim for more shorter exposures that fewer longer exposures (more of which sometimes get rejected).
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