  In order to keep pace with the increasing data quality of astronomical
surveys the observed source redshift has to be modeled beyond the well-known
Doppler contribution. In this letter I want to examine the gauge issue that is
often glossed over when one assigns a perturbed redshift to simulated data
generated with a Newtonian N-body code. A careful analysis reveals the presence
of a correction term that has so far been neglected. It is roughly proportional
to the observed length scale divided by the Hubble scale and therefore
suppressed inside the horizon. However, on gigaparsec scales it can be
comparable to the gravitational redshift and hence amounts to an important
relativistic effect.
