Background derivation (Sect. 3.2) for ℐ_ƛ ≡ 𝒟_13″ from Eq. (8). The left panels show the backgrounds ℬ_ƛX and ℬ_ƛY, obtained using the procedure described by Eqs. (20)–(22). The middle panels show the corresponding background-subtracted 𝒮_ƛ and ℱ_ƛ from Eq. (23). The right panels show the relative errors of ℬ_ƛX and ℬ_ƛY with respect to the true model backgrounds 𝒟_C and 𝒟_B (Fig. 2), convolved to the same resolution. The filament is heavily blended with itself in the central area, therefore its background is systematically underestimated there (lower right). Square-root color mapping, except in the right panels, which show linear mapping.