Background evaluation in the eRASS1 simulation. Top panel: comparison between the mock and real background maps. The lines identify the number of pixels showing a given value of the background map. The mock data is denoted in blue, the real eRASS1 in orange. The dashed blue line shows the simulated background re-scaled by 0.9. The lower panel shows the ratio between the mock and real data. Bottom panels: impact of a 10% overestimation of the background on the analytically computed value of detection likelihood. The left-hand panel shows DET_LIKE as a function of counts in each pixel given by a source and by the background. The panel on the right shows the corresponding percentage error on detection likelihood caused by a 10% larger background. The blue and the green solid lines respectively denote DET_LIKE = 5 and 20.