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Signal-to-Noise Ratio

The signal-to-noise ratio of an observation is:

s/n = $${\frac{Ct}{\sqrt{\sigma^2 n + t(C-AS)}}}$$

where C is the count rate from the object (photons per second), t the exposure time, $\sigma$ the read-out noise of each pixel, n the number of pixels on which the image falls, S the number of photons from the sky per unit solid angle, and A the solid angle subtended by the image.