Bayesian SED-matched source detection with a three-SED prior. The prior is that astronomical sources are a mixture of 49% “Red” (color g − r ≃ 1 mag), 49% “Flat” (color g − r = 0 mag), and 2% r-band only (g flux is zero). Left: probability contours for the foreground model are extended in the directions of the three components of the prior. The background model (null hypothesis) is that there is no source, only noise; these contours are circles centered on zero. The probability contours are spaced in powers of 10. Right: probability ratio contours for the foreground divided by background model. When performing source detection, these lines mark decision boundaries; points above and to the right of a decision boundary are considered to be detected. The contours are spaced in powers of 10.