Three different random draws of time-correlated systematics for a grid of 3 × 3 pixels. Left grid shows flux-conserved systematics whose sum is zero after aperture extraction. The second grid shows systematics independent to a particular pixel. The third grid shows common systematics with the same shape and with an amplitude proportional to the flux on each pixel. While the amplitude of common systematics may appear small, their effect combines together over multiple pixels to reach a similar amplitude to the independent pixel systematics after aperture extraction.