Achievable signal-to-noise for the BAO detection as a function of limiting flux and sky coverage, shown with numbered contour lines. From top to bottom, three different choices for the quality of optical follow-up are considered. The diagonal solid lines show possible survey size-depth relations for several existing and proposed X-ray instruments assuming a total observation time of 100 days (lefthand panels) and 1000 days (righthand panels). Redshift range z = 0.5 − 2.5 is assumed. For eROSITA and XMM-Newton, the small vertical lines at F_lim ≃ 2 × 10^-15 and 7 × 10^-16 erg s^-1/cm² mark approximate confusion noise limits for these instruments. For the other telescopes, confusion noise is not achieved in the considered flux range.