Model of the PUMA spectrum. The Hα plus the two [NII]6548, 6583 lines, transmitted by the filter, are represented by Gaussian functions of width σ_LSF (see Table 1). Normalised units to the Hα line are used for the y axis. The relative intensity between the Hα and the [NII] lines are given in Sect. A.1. The distance between the two green dashed vertical lines represents the FSR ( ∼ 20 Å). Top panel: Redshifted Hα and [NII]6548, 6583 lines are located at their right position. The distance between the two grey dashed vertical lines represents the filter width (FWHM ∼ 90 Å). Bottom panel: Redshifted lines located at their apparent position due to the FSR recovering. The [NII]6548 line is shifted by plus one FSR, and the [NII]6583 is shifted by minus one FSR. The green curve represents the sum of the Hα plus [NII]6583 lines, whereas the red one represents the Gaussian that best fits the sum of the two lines (best fit method).