Example time step of the test with synthetic data. Top panel, left column: synthetic flow map in Plate Carree coordinates for v_lat (top) and v_lon (bottom). Middle: after subtracting the Zernike fits and limiting the field of view to 60° from disk center. Right: sum over the Zernike fits. The red lines indicate the positions of the line plots in the bottom panel. Bottom panel: line plots showing the synthetic data before (black) and after (blue) subtraction of the Zernike fits for v_lat (left) and v_lon (right).