Model Faraday thickness of a slab along the line of sight towards the Perseus cluster as a function of the distance to the near side of the slab. The far side of the slab is held constant at 20 kpc distance from the Sun. The assumed magnetic pitch angle is 12° (solid lines) or 8° (dashed lines). The thinner sets of lines represent magnetic models where the total field strength was multiplied with a factor of two or four. The grey bar indicates the 1σ and 2σ uncertainty levels of the observed Faraday thickness of the slab between the high-φ polarized emission and the polarized background sources.