Problem geometry and schematic of statistical fluctuation cancellation for an observation of an ICM at Perseus redshift through the X-IFU, which observes the plane of the sky (e_y, e_z). The coloured boxes can represent either temperature, density or velocity fluctuations. The colour scale is arbitrary but blue and red tones are of opposite signs, and the more opaque the stronger the fluctuation. The dependency of the fluctuations with y, z is overlooked for clarity.