Figure 2

Celestial sphere and black hole shadow images for an observer located at \(r=10~r_{\mathrm { g}}\). Top panel: celestial sphere and shadow image as seen by a stationary observer. The different colors represent different quadrants of the sky, yellow and blue being behind the observer, while red and green are in front of the observer. The black lines represent lines of constant longitude and lattitude while the black, circular region in the center is the black-hole shadow. Middle panel: as top panel, but seen by a radially in-falling observer. Bottom-left panel: photons originating from a stationary observer’s camera, as used to generate the top panel. Bottom-right panel: photons originating from a radially in-falling observer’s camera, as used to generate the middle panel. The black hole event horizon is shown as the black region in both bottom panels. The shadow sizes are similar in both panels, but differences are clearly visible. See corresponding text for further discussion