Molecular maps of H₂O for real PZ Tel B data using CCS. This figure shows a real case example where the noise structures may reduce detection capabilities of cross-correlation methods. The brown dwarf was observed under good conditions (airmass: 1.11, Seeing start to end: 0.77–0.72) and lower conditions (airmass: 1.12, Seeing: 1.73-1.54), cf. Appendix A for full details on observing conditions. Upper plots show molecular maps of PZ Tel B, while the lower plots show the cross-correlation series along the radial velocity (RV) support for pixels at the centre of the object, and within the object’s brightness area. While the brown dwarf should appear at the same spatial coordinates for respective RV locations in both cases (cf. vertical lines), it is clearly visible when conditions are good, but hardly visible on equal scales under lower conditions.