= Poisson variance after sky subtraction
{c}
{title2=$R=\frac S{\sqrt{S+B+B/n_b}}$}
Subtracting an exactly known mean sky level leaves source-plus-sky <variance> of a <Poisson distribution> $S+B$. If the sky per source <pixel> is estimated from $n_b$ equal-area, equally exposed independent sky <pixels>, its additional <variance> is $B/n_b$, so $R=S/\sqrt{S+B+B/n_b}$ before read and dark terms. The <variance> of removed background <photons> does not disappear with their mean.
Back to article page
