Flux density ratio computed for the Hα emission line observed in the lensed quasar HE0435−1223. This ratio is illustrated as a function of the Doppler velocity over the useful [ − 8600, 8600] km s⁻¹ velocity range (red lines). The continuum-subtracted spectra of images B and D (M ×  and , respectively) are superimposed, on an arbitrary flux scale (thin black and blue lines). These spectra were corrected for instrumental spikes and smoothed using a 220 km s⁻¹ -wide median filter in the velocity range of interest (thick green and magenta lines). μ(v) is shown with the 220 km s⁻¹ smoothing and a stronger one using a 1500 km s⁻¹-wide median filter.