An innovative concept of wavefront sensing for Rayleigh beacons is introduc
ed along with an example of a possible wavefront sensor. This new approach
does not require the gating technique to limit the useful range of the lase
r source and therefore looks simpler to implement than previous Rayleigh co
ncepts, and may additionally allow more efficient use of the photons emitte
d by the Rayleigh beacon. Our technique is based upon an optical element in
the focal plane area whose section does not change for the conjugation of
different ranges from the telescope aperture, hence the name z-invariant. T
he wavefront sensor shown here is an example of this new class. It is a com
pact pupil-plane wavefront sensor and as such allows for a layer-oriented c
onfiguration. It is shown that its sensitivity, while higher than usual gat
ing approaches, is far from the possible limits leading us to speculate tha
t other z-invariant wavefront sensors can reach much larger efficiencies.