Upper panel: atmospheric extinction, K(λ) (solid line), for a given non-photometric night (2010-06-28); i.e. the sum of its physical components, Rayleigh scattering k_R(λ) (dashed line), aerosol scattering k_A(λ) (dash-dot line) and ozone absorption (dotted line). Middle panel: instrument calibration C(λ); this includes not only the overall throughput, but also scaling by the spectral flat field. Bottom panel: relative error of the standard star fluxes after the fit (i.e. the difference between the individual standard star flux solutions and log C(λ)). The grey bands in all three panels indicate meta-slices that are affected by the “strong” H₂O telluric features. This wavelength region is not used when fitting for the other physical components.