Red lines show the adopted measurement error model as a function of the nominal total [0.5–2] keV CR and apparent r_c; the black circles are the detected Aardvark C1 clusters, drawn to highlight the cluster locus in this parameter space. Practically, the error on CR and r_c are randomly ascribed from a log-normal distribution with the dispersion given by the red abacuses. Errors on HR₁ and HR₂ are assumed to be respectively the same and the double values obtained for the corresponding [CR, r_c] combination. The model assumes a mean vignetting value.