Signal-to-noise ratio of the tomographic signal as a function of the fraction of sky f_sky covered in the survey and A_eff × t – a product of the effective area A_eff, at the energy where the tomographic signal is extracted, and the time t spent per field. The dashed S/N contours correspond to values 1, 2, 4, 8, 16, 32, and 64. The straight lines show the locus of the points, which can be achieved in the course of a 1-year survey (4 years for eROSITA) by various currently existing, near-term, and possible future X-ray instruments. The panels show results for different values of the 6.4 keV line equivalent width: 500 eV (top panel), 300 eV (middle) and 100 eV (bottom). Observational energy bands 3.0−3.2 keV and 3.3−3.5 keV, corresponding to z ≃ 1, are assumed.