Gaia-ESO Survey bulge selection function. The background Hess diagram depicts a generic CMD in the bulge region from VVV photometry. Prominent sequences are labeled. A shaded white area indicates the main selection function, with color and magnitude cuts of (J−K_s)₀ > 0.38 and 12.9 < J₀ < 14.1, respectively. The green shaded area indicates the magnitude extension implemented in fields where the double red clump feature is visible. The whole spectroscopic sample analyzed in the present study is displayed with black dots.