Long-slit spectra of the A and B components of Q0957+561. Panel a: linearized source spectrum. Panel b: sky subtracted spectrum with removed cosmic rays. Panel c: extracted spectra of components A (blue) and B (red) of the lensed quasar image. Panel d: brightness ratio of the components. Panel e: spectra of components A (blue) and B (red) after continuum subtraction. Panel f: spectrum of the lensing galaxy in arbitrary units and the comparison star, shifted by z = 0.36. Panel f (left): also shows the cross-correlation function between the spectra of the star and the lens galaxy.