Footprint expansion, illustrated in an image with 3″ resolution of a source with a peak intensity of 100, half-maximum size of 10″, and S/N of 100 (left). The source has an intensity profile defined by Eq. (2) with Θ = 5″ and ζ = 1, transforming into a power law I ∝ θ⁻² for θ ≫ Θ and filling up the entire image, its faint outer areas (I ∼ 0.2) are largely lost within the noise. The initial footprint factor ϕ_n = 3 (Sect. 3.4.4) is too small for these power-law sources, hence background subtraction leaves a relatively bright pedestal containing a large amount of the source emission (middle). The footprint expansion algorithm (Sect. 3.4.6) enlarges ϕ_n by a factor of 2.2 (right), which lowers the source background by a factor of 5, and as a result, increases the source flux by a factor of 2.7. The improved flux is still below the true value by a factor of 1.9 because the actual footprint is about three times larger. Square-root color mapping.