NTT/EFOSC2 optical spectra of the two components of the lensed quasar eRASS1 J050129.5−073309. Top panel: black and red curves correspond to image A and image B of the doubly lensed quasar. The gray curve corresponds to the central Moffat profile fitting of the 2D spectrum. The flux of this spectrum is increased by a factor of 15 for visualization purposes. We show observed and rest-frame wavelengths to illustrate the redshift of the source. We annotate the main emission lines recognized in the spectra. Middle panel: ratio between the spectra from image A and image B. Bottom panel: subtraction of scaled spectrum B from spectrum A using a scaling factor of α = 2.4 (see Sect. 3.1 for more details). The vertical purple and green lines highlight the main emission and absorption lines, respectively. The velocities of the absorption features are −3460 km s⁻¹ and −6320 km s⁻¹ with respect to z = 2.47.