Example of global line fit of the line Fe Iλ_lab 5379.5740 Å described in Sect. 5. Top-left panel: normalised flux of a HARPS-LFC spectrum (black thin line) of the Moon, labelled s429 in Table 1, centred on λ_lab = 5379.5740 Å as a vertical green dashed-dotted line. Top-right panel: observed line profile together with unbroadened 3D line profiles of three different strengths. Bottom-left panel: global line fit (blue dashed-dotted line) to the line flux points within the spectral region of 0.4 Å (region within the vertical red dashed-dotted lines) centred on the laboratory wavelength (vertical green dashed-dotted line). Bottom-right panel: line core flux points (black dots) and global line fit (blue dashed-dotted line) with the laboratory wavelength (vertical green dashed-dotted line) shown as reference.