  The shape of a focus-modulated point spread function (PSF) is used as a quick
visual assessment tool of aberration modes in the PSF. Further analysis in
terms of shape moments can permit quantifying the modal coefficients with an
accuracy comparable to that of typical wavefront sensors. In this letter, the
error of the moment-based wavefront sensing is analytically described in terms
of the pixelation and photon/readout noise. All components highly depend on the
(unknown) PSF shape, but can be estimated from the measured PSF sampled at a
reasonable spatial resolution and photon count. Numerical simulations verified
that the models consistently predicted the behavior of the modal estimation
error of the moment-based wavefront sensing.
