Illustration of the degeneracy between T and β in a SPIRE colour–colour plot. The solid lines correspond to constant values of β or T. The lengths of the lines correspond to a temperature range of 10–23 K or a spectral index range of 1.0–2.5, respectively. Four distinct parameter pairs are plotted as red dots. The surrounding contours indicate the FWHM extent of the estimated parameter distributions in case of 1% or 5% uncertainty in all surface brightness data.