English: Polar plot (using the receiver-frame rather than the source-frame angle) of received/source Doppler frequency-ratio (greater than 1 in the leftward approaching-direction, less than 1 in the rightward receding-direction) for light in a vacuum.

The coordinate-speed (v/c) color-key (for values between 0 and 0.9 in steps of 0.1) is indicated with colored dots (red means v/c=0, cyan means v/c=0.5, magenta means v/c=0.9) along a line in the approaching-source (leftward blue-shift or increased-frequency) direction. The curves are ellipses with the origin at one focus, eccentricity v/c, and semi-major axis γ = 1/Sqrt[1-(v/c)²].

The transverse red-shift, seen when we use receiver-frame right-angles, is mapped with black dots along the vertical-axis through the origin. The transverse blue-shift, seen when one considers source-frame right-angle events instead, may be found marked with white dots at the semi-minor axis points along each oval.

This figure is complementary to a source-frame angle figure also posted here.