  In this paper we introduce the `redshift fluctuation' as a gauge-invariant
cosmological observable and give its fully relativistic expression at first
order in cosmological perturbation theory. We show that this corresponds
effectively to number counts with a radial window function with vanishing mean
which therefore resolve smaller scale radial modes than standard number counts.
In a detailed analysis of the angular power spectrum of this new variable, we
study the relevance of different relativistic contributions, and how it differs
from the conventional observable galaxy number count fluctuations. In order to
investigate its utility for future spectroscopic surveys, we perform Fisher
forecasts for a Euclid-like and an SKAII-like configuration, as examples.
Particular focus is placed on the dependence of the results on the size of the
redshift bins and on the cutoff in $\ell$ adopted in the analysis.
