Top: MBB parameters fitted to the background, with T(K) on the left and β on the right. The high-temperature region at (l = 155° ;b = 77°) is artificially created by strong artefacts in the IRIS data. Middle: inverse relative uncertainty T/ΔT (left) and β/Δβ (right) of the MBB parameters. Bottom left: reference background brightness, log(Jy pixel⁻¹, evaluated at 857 GHz. This is the value at 857 GHz of a multifrequency fit, not the value directly measured at 857 GHz. Bottom right: log(RELTH), computed with α = 5% (see Eq. (9)). On all panels there is a region at (l = 208° ;b = −18°) of extreme values and uncertainties caused by artefacts present in the IRIS data. The colour bars have been histogram-equalized. regions are either inside the IRIS mask, or had insufficient data.