Cartoon of a 90° azimuth (east–west) cut through two adjacent primary beams of any wide-field telescope to compare with the systemic difference shown in Figure 7. When the model of the primary beam (solid black) is applied in place of the true model (dashed) the error is manifested as a characteristic flux scale that varies with position (dashed lines). When two pointings are differenced, the errors on the opposing edges will have opposite signs. The effect will be most pronounced when comparing sources occurring at the extremes of both beams along the axis bisecting both facets (here right ascension).