The signal-to-noise ratio,
is given by
This equation assumes the optimistic (and often realistic) condition that the background zero point level under the object is sufficiently well known (and subtracted) to not significantly contribute; in crowded fields this may not be true.
Observers using the CCD normally take sufficiently long integrations that the CCD read noise is not important. This condition is met when:
For the CCD in the regime where read noise is not important and for all SBC observations, the integration time to reach a signal-to-noise ratio
, is given by:
If your source count rate is much brighter than the sky plus detector backgrounds, then this expression reduces further to:
i.e. the usual result for Poisson statistics of
.
More generally, the required integration time to reach a signal to noise ratio
is given by:
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