Mean antenna temperature as a function of frequency, measured outside the union mask before (red points) and after (black points) offset corrections. The dashed lines shows the best-fit sum of a low-frequency power-law (with free spectral index) and a one-component greybody (with fixed emissivity of β_d = 1.6 and temperature T_K = 18 K; Planck Collaboration XI 2013; Planck Collaboration XII 2013; Planck Collaboration Int. XVII 2014; Planck Collaboration Int. XXII 2014) to each of the two cases.