Schematic representations of common defects on the prism apex and their impact on quadrant positioning. Panel a: nominal design of the pyramid apex: quadrants lie on a perfect square, although the side length of the square needs not match an integer multiple of the pixel size. Panel b: polishing angle of facet B is excessive, shifting outward edges of facet B; quadrant B is moved away from the optical axis and receives more flux, whereas A and D receive less. Panel c: facet polishing depth is unequal, creating a roof apex. Intensities at zero-phase point are modified.