Comparison of the GMOS 3′′ aperture spectra (black line) with the corresponding SDSS spectrum (red line). The wavelength range is limited to the rest-frame wavelength range between 4500 and 5500 Å. An absolute photometric calibration of the GMOS data based on the median of the ratio of spectra over the common wavelength range. The spectra are normalized in flux density so that the peak in the [O iii] λ5007 line is set to one in the SDSS spectrum. A large part of the SDSS spectrum of SDSS J0304+0022 is masked as bad, as seen as the linear interpolated region. In general, the relative GMOS flux calibration across the wavelength range is consistent with the SDSS spectra at a <10% level.