Example spectra (red solid lines) of (a, b) HB9 extracted on 2024.03.29 at the position of the acquisition star (d = 0, No.= 25; Fig. 1) and (c, d) G159.2+3.3 extracted on 2024.03.27 at d = −2.25′ from the acquisition star (No.= 15; Fig. 2). The grey solid curve is the best-fit to the spectra with Gaussian models plus a linear continuum. The lower panels show the spectral fit residuals defined as (model - data)/error. The slight difference in the residual continuum level in the upper and lower panels is caused by the imperfect local background modelling and subtraction, which however, does not affect the modelling of the emission lines.