The correction spectrum between DR7 and DR9 derived from 80 F-type stars (gray line). The smooth solid black line represents the four-order polynomial fit, i.e., Equation (1), which was used to correct the systematic difference between DR7 and DR9. The blue and red lines represent the correction spectra constructed with 19 bright and 61 faint F-stars, respectively (see the text for details); they are smoothed every 50 points to help distinguish them from the overall correction spectrum.