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Abstract

We show that, away from the axis of symmetry, no continuous degeneration
exists between the shadows of observers at any point in the exterior region of
any Kerr-Newman black hole spacetime of unit mass. Therefore, except possibly
for discrete changes, an observer can, by measuring the black holes shadow,
determine the angular momentum and the charge of the black hole under
observation, as well as the observer's radial position and angle of elevation
above the equatorial plane. Furthermore, his/her relative velocity compared to
a standard observer can also be measured. On the other hand, the black hole
shadow does not allow for a full parameter resolution in the case of a
Kerr-Newman-Taub-NUT black hole, as a continuous degeneration relating specific
angular momentum, electric charge, NUT charge and elevation angle exists in
this case.