Selection of four apertures to study the missing flux in the HCN and CO maps of NGC 1068. Upper panel: Overlay of the four ∼30″ size apertures (black circles, identified by the numerical labels 1-4) selected to cover the SB ring of NGC 1068 on the ALMA HCN(1–0) map (colour scale and contours as in Fig. 1), used in Appendix A to estimate the missing flux correction. Lower panel: Same as the upper panel but showing the overlay of the four ∼12″ size apertures (black circles, identified by the numerical labels 1-4) selected to cover the SB ring of the galaxy on the PdBI CO(1–0) map of Schinnerer et al. (2000). The colour scale is given in Jy km s⁻¹ beam⁻¹ units and the position of the AGN is identified by the black star markers in both maps. The magenta filled ellipses at the bottom-left in the two panels represent the beam sizes.