The visibility of several hypothetical candidates during a night is shown in the upper panel. The lower panel demonstrates how the visibility factor x_vis for each of these candidates changes as a function of time. The x_vis definition from the main body of the paper (dashed line) has no upper limit as candidates set below a_min. The modified version (Eq. B.2, solid line) ‘saturates’ to a maximum value. For this example, we have chosen A = 10 and p = 10, and a_ref = 90°. The white region is when the Sun is below altitude a < −18° (astronomical night). Open circles mark when the time the candidate sets below the minimum acceptable altitude, a_min, or the sun rises above a = −18° (whichever is earlier).