Left: Histogram of the background values in randomly placed annuli on the field images with polynomial sky subtractions of order 0 (blue), 1 (purple), and 2 (green). The vertical lines denote the corresponding 1σ values, as shown in the upper right corner. The scatter to the background values is smaller for higher-order polynomial estimation of the background. It is clear that higher-order polynomials remove spatial variation in the background more efficiently. Right: Mean and scatter of the mean background values for the different polynomial background estimations (indicated in the upper right corner). This again demonstrates how the scatter is gradually reduced for higher-order polynomial estimation to the background values.