Corner plot showing parameter correlations for three areas in the fits of Fig. D.1. The parameters are the optical depth τ, the mean temperature T_C, and the standard deviation of the temperature distribution δΤ_C. The data correspond to averages over 3 × 3 pixels towards the centre of the coldest source (blue), a position in the 15 Κ background (black), and the centre of the warmest source (red colour). The contours are drawn at 10%, 50%, and 90% of the peak probability density.