(a) Schematic illustration of the Radon transform. (b, c) Close-up views near the stellar limb during ingress and egress, respectively. In the knife edge approximation, the curved stellar limb is locally approximated by a straight edge; the dashed lines indicate the locally linearized limb at different times. The blue arrows indicate the local inward normals to the stellar limb. In the Radon transform limit, these normals define the projection-normal direction.