The effect of the flux prior in Bayesian SED-matched source detection. In order to produce closed-form integrals, we use an exponential prior on the flux; here we show the effect of the exponential scale factor (α) on the results. Left: probability contours for the “Red” foreground model, as before, with α = 1, compared with a flux prior that prefers brighter sources—larger observed fluxes are considered more probable—with α = 0.5. Right: a one-dimensional example showing the effect of changing the flux prior by a factor of two. The dashed model places more prior belief in larger fluxes and therefore must produce smaller probabilities at low flux levels. This illustrates that care must be taken with the scale factor of the flux prior.