Observed and simulated noise maps (top) and their respective auto-correlation functions (bottom). The real [C II] map is displayed in the top left corner, with the source masked in the center. For comparison, we also display the [C II] dirty beam in the middle of the bottom panel. All the images on each row are displayed with the same color bar. The auto-correlation functions were rescaled prior to display to a peak value of unity. For easier comparison, we also show a circular average of the covariance in the central column of the first row, with the observed (black) and simulated (green dashed) auto-correlation functions, and the dirty-beam profile (blue).