Comparison of the sky-subtraction methods for two example stars (marked in Fig. 5 with the corresponding colors). The top panel gives the spectra of an example bright star (star 152, a Be star with V = 17.15 mag), and the bottom panel gives those of a fainter star (star 111, a B star with V = 17.51 mag). We show for both stars the original spectrum without any sky subtraction (no sky subtraction), the spectrum with the sky subtraction deduced from the dedicated sky cube (sky cube), and the spectrum with the sky subtraction determined in the science data cube (data cube). The spectra are not corrected for telluric lines (see, e.g., the region between 6850 and 7000 Å).