Illustration of our masking procedure. Upper row: original CFIS image in the r band, followed by the image after applying the LoG filter in the top middle panel. This image is then thresholded and all pixels below 6σ_sky are set to zero, as shown in the top right panel. Bottom row: detected peaks, whose centroids are marked with black crosses. Bottom middle panel: estimated lens size as a black circle and the red areas indicate what we consider as contaminants. These are masked as the red area on the bottom right image.