Periodograms of a spectrum of NGC 7027 before (top) and after (middle) fringe flat correction and after residual fringe correction (bottom) obtained from a single spaxel of band 2A. The frequency of the main peak matches that derived for the fringes and its amplitude decreases after applying the fringe corrections, which indicates that the fringes have been reduced. The region with frequencies between 10 and 20 mm is nearly flat and is not shown.