Illustration of the decontamination procedure for a line-emitting source. Top: Original RGS000+0 spectrogram of object ID 2709725257636288279 in SIR coordinates: the x-axis is the dispersion direction, and the y-axis is the cross-dispersion direction. Middle: Model for the bright contaminant created in this case from broadband photometry, which affects a part of the target spectrogram. Bottom: Decontaminated spectrogram of the source of interest (positioned along the white dashed line). The flux scaling of all panels is the same. We note that the decontamination procedure was successful at isolating the target spectrogram, even if a faint subtraction residual is still visible in this particular case.