Comparison of the direct (empirical) and model-predicted completeness maps, illustrated for the globular cluster NGC 1261. Top left: map ratio of the number of sources in Gaia and HST in the magnitude range 20.9 < G < 21.1. Top left: map of M₁₀ used to predict the completeness. Bottom left: predicted completeness at G = 21. Bottom right: map of the difference between the predicted and observed completeness: the variance is largest in the intermediate completeness regime (see Fig. 5), producing a ring-like structure in the residuals map.