Upper panel: Histogram of the normalised deviation of the derived spectral index with respect to the true input spectral index $({\beta }_s^{\mathrm{Inferred}}-{\beta }_s^{\mathrm{Input}})/{\sigma }_{{\beta }_s}^{\mathrm{stat}+\mathrm{sys}}$ (orange). The black curve is a Gaussian distribution with zero mean and unit variance. Vertical lines are the corresponding mean of the distribution. Lower panel: The two-dimensional density plot of true input and inferred β_s map. The correlation between the two maps is 81%. The black dashed line correspond to 100% correlation.