Left panel: simulated flat-field structure using Perlin and pixel-to-pixel Gaussian noise. Right panel: example of one of the simulated precalibrated frames ( × ) we used for the construction of the sky flats. White regions represent the source masks. Bias and dark corrections were applied, but no flat-field correction was performed, showing the same background structure as the original flat field in the left panel. See the color bar at the bottom.