Top plot: comparison between FM extended source spectrum and spectrum derived from summing multiple point sources in the MRS FOV. The two spectra have a different spectral baseline, caused by the different spectral energy distribution of the sources. Middle plot: comparison of the fringe transmission derived from analyzing the spectra in the top plot. Bottom plot: ratio of the fringe transmission profiles of the middle plot. The mean and standard deviation of the ratio are delineated with the red dashed line and the black continuous lines, respectively.