Results from a radiative transfer model resembling the CrA C core. Frame a) the ratio of the colour temperature (derived from synthetic surface brightness observations) and the mass-averaged real dust temperature. Frame b) the derived values of τ_{250   μm} as the function of the true optical depth in the model. The dotted box corresponds to the range of optical depths in Fig. 7. The dashed green line represents a 1:1 correlation. The red curve represents an exponential function fitted to the data points (see the text).