Figure 5

Figure 5: Distortion pattern. The picture shows, in a Mercator projection with P as the horizontal and Q as the vertical coordinate, the celestial sphere of an observer at a spacetime point where the Weyl tensor is of Petrov type N. The pattern indicates the elliptical images of spherical objects to within lowest non-trivial order with respect to distance. The length of each line segment is a measure for the eccentricity of the elliptical image, the direction of the line segment indicates its major axis. The distortion effect vanishes at the north pole Q = 0 which corresponds to the fourfold principal null direction. Contrary to the other Petrov types, for type N the pattern is universal up to an overall scaling factor. The picture is taken from [56] where the distortion patterns for the other Petrov types are given as well.