Quote:
Originally Posted by Placidus
Introduction
Someone else might like to write an equation for signal to noise ratio that includes sky glow.
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FWIW, suggest that the equation for sky limited imaging is:
SNR = N*Q*T*P / sqrt(N*Q*T*P + N*Q*T*S + N*T*D + N*RN*RN)
where S is the sky signal and D the dark current
the generally agreed setting for sub length is to make N*Q*T*S the dominant term in the denominator, at which point the SNR is independent of RN (ie there is no gain in SNR from using short subs).
Normally, subs will be long enough that N*Q*T*S >~ 10*N*RN*RN - or something similar. ie, N cancels out and the required sub length T scales directly with RN*RN, the same as for narrowband. Since sky-limited exposures with normal cameras come in at 10-20 minutes for a 10e RN camera, an equivalent system with a 2e RN camera could do the same job with 24-48 second subs.