Figure 5: Distortion pattern. The picture
shows, in a Mercator projection with as the horizontal and as the
vertical coordinate, the celestial sphere of an observer at a
spacetime point where the Weyl tensor
is of Petrov type . 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 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.
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