Airmass variation and S/N of an ESPRESSO spectral order covering wavelengths from 567.76 nm to 576.42 nm, chosen as a midpoint in the selected Fe I spectral lines, as a function of the orbital phase of HD 189733 b. Blue points correspond to observations from 11 August 2021, and red points to those from 31 August 2021. The darker shaded region marks the phases where the planet is fully in transit, with lighter shading indicating the ingress and egress phases.