X-ray flux for a saturated coronal source at a distance of 120 pc vs. effective temperature (red data points) and apparent Gaia G magnitude for the same objects (blue data points). The dash-dotted lines refer (from top to bottom) to limiting flux levels for the ROSAT all-sky survey, the minimum eROSITA all-sky survey, and to a hypothetical limiting eROSITA flux at high ecliptic latitudes. See text for details.