Left: example of the SIXTE simulated image of an 80 ks ATHENA/WFI observation. Cosmic X-ray background, Galactic foreground, and particle background are included as well as telescope vignetting and PSF degradation with an off-axis angle. The blue circles on the left correspond to the size of r₅₀₀ of the input groups, which are located at z = 2.0 with M₅₀₀ = 5 × 10¹³ M_⊙. Right: wavelet image of this SIXTE simulated image. The blue circles represent the detected sources after SEXTRACTOR and the maximum likelihood fitting detection algorithm are run; three sources are undetected by our algorithm.