Results of fitting our narrow-band bootstrapped images to Eq. (6). For all images the contours overlaid are at 70 MHz. Top left: best-fit covering fraction, f, per pixel. No information about the location of the reverse shock is fed to the fit, but it naturally recovers f = 1 for regions outside the reverse-shock radius (i.e. no internal absorption). The average value of f inside the reverse shock is 0.78. Top right: deviation from power-law behaviour. This plot corresponds to (f + (1 − f)e^−τ_ν,int) for our best-fit values of f and X (see Eqs. (5) and (7)). Bottom left: best-fit S₀ per pixel. This corresponds to the flux density of Cas A at 1 GHz in jansky if no absorption were present. Bottom right: reduced χ² of our fit.