Cumulative flux profiles for a point source with the average RASS PSF (dotted line), a β-model profile with a core radius of 1 arcmin (dashed line), and the same β-model profile convolved with the PSF (solid line). The profiles are normalized to unity at infinity. The observed flux fraction is read off from the solid line at r_out and the expected flux fraction at r₅₀₀ is marked as f₅₀₀. The ratio of the two values gives the correction factor for the missing flux.