$\propto (\lambda / \lambda_{\mathrm{ref}})^{-0.2}$, normalized to the value measured on each observed spectrum at the effective wavelength of the $R$ filter used for the centering of the target ($\lambda_{\mathrm{ref}} =6600$ Å).\
For each spectrum, the distance of the PSF centre to the slit centre at a given wavelength was assumed to be the differential refraction between this wavelength and $\lambda_{\mathrm{ref}}$ multiplied by the cosine of the difference between the parallactic angle at the time of the observation and the parallactic angle at the time when the object would be at an airmass of 1.41, converted to degrees ($25^{\circ}$ and $18^{\circ}$). Here we assumed that the Longitudinal Atmospheric Dispersion Corrector (LADC) of FORS2 [@Avila97] performs optimally up to airmass of 1.41. For the calculation of the differential refraction, we assumed the usual atmospheric conditions at Paranal (temperature $ T = 12\,^{\circ}\mathrm{C}$, pressure $P = 743$ mbar). The integrated value of the flux along the spatial direction entering the spectrograph can then be calculated for each wavelength. The factor representing the slit throughput is then the ratio between this integrated value and the total flux at that wavelength. The top panel of Fig. \[fig1\] shows the estimated slit throughputs as a function 