NuSTAR FPMA image of 3C 111 (ObsID 60202061002) as an example. The left panel shows a circle of 60″ for source extraction. The right panel shows source-free regions used for background simulation in NUSKYBGD that consist of four rectangular regions excluding a circle of 120″ centered on the source. The image in the right panel is smoothed to demonstrate the spatially nonuniform background.