  Thanks to the availability of high-resolution high-sensitivity telescopes
such as the Very Large Array, the Hubble Space Telescope, and the Chandra X-ray
Observatory, there is now a wealth of observational data on relativistic jets
from active galactic nuclei (AGN) as well as galactic sources such as
Black-Hole X-ray Binaries. Since the jet speeds cannot be constrained well from
observations, but are generally believed to be relativistic, physical
quantities inferred from observables are commonly expressed in terms of the
unknown beaming parameters: the bulk Lorentz factor and the line-of-sight
angle, usually in their combination as relativistic Doppler factor. This paper
aims to resolve the discrepancies existing in the literature about such
"de-beaming" of derived quantities, in particular regarding the minimum-energy
magnetic field estimate. The discrepancies arise because the distinction is not
normally made between the case of a fixed source observed with different
beaming parameters and the case where the source projection on the sky is held
fixed. The former is usually considered, but it is the latter that corresponds
to interpreting actual jet observations. Furthermore, attention is drawn to the
fact that apparent superluminal motion has a spatial corollary, here called
"retardation magnification", which implies that most parts of a relativistic
jet that are actually present in the observer's frame (a "world map" in
relativity terminology) are in fact hidden on the observer's image (the "world
picture" in general, or "supersnapshot" in the special case of astronomy).
