  Dark wavefront sensing in its simplest and more crude form is a quad-cell
with a round spot of dark ink acting as occulting disk at the center. This
sensor exhibits fainter limiting magnitude than a conventional quad-cell,
providing that the size of the occulting disk is slightly smaller than the size
of the spot and smaller than the residual jitter movement in closed loop. We
present simulations focusing a generic Adaptive Optics system using Natural
Guide Stars to provide the tip-tilt signal. We consider a jitter spectrum of
the residual correction including amplitudes exceeding the dark disk size.
