An example of negative correlation between colour temperature and spectral index. The field is G300.86-9.00 in Musca. The frames, as indicated by their colour scales, show 250 μm surface brightness, τ(250 μm) optical depth, colour temperature T, and the spectral index β. The bottom frames show the β maps that correspond to the 16% and 84% percentiles in a Monte Carlo study of zero point errors (see text). The maps are based on SPIRE data and, for plotting only, they have been convolved to a resolution of 1′.