H − K colours of stars in the background of CrA C as a function of a modified τ_250  μm where we have attempted to correct for the effect of line-of-sight temperature variations. It is assumed that the relationship between the observed and “true” optical depth is similar to the model shown in Fig. A.1. The red curve shows the relationship expected from the result of Roy et al. (2013) where the σ^H increases as and the green line is a linear fit to the data points. The mean error of the data points is indicated with a cross in the bottom right.