Atomic homodyne detection allows the field characteristic function to
be reconstructed from the excitation probabilities of probe atoms that
interact both with the prepared cavity field and a reference driving
field. We examine the effects of retaining a full quantum description
of both fields and show that the homodyne approximation limits the tim
e of observation of the field. This in turn limits our ability to reco
nstruct precisely the field characteristics from the restricted data,
and can make it impossible to determine fine quantum detail, and, in p
articular, to discriminate between field superpositions and statistica
l mixtures.