Fit of (T, β) in the field G300.86-9.00 using IRAS and Planck data. The frames a), c), and d) show 250 μm surface brightness, colour temperature, and spectral index fitted to 3000 GHz (IRAS 100 μm), 857 GHz, 545 GHz, and 353 GHz data, respectively. Frame b) shows, for comparison, the higher resolution Herschel τ(250 μm) map. Frame e) is the surface brightness residual between the 217 GHz observation and the above fit. The last frame shows the difference between spectral indices derived with 3000–353 GHz and 3000–217 GHz data. If β decreases with wavelength, the residuals in frames e) and f) will be positive.