  The effects of photon noise, aliasing, wavefront chromaticity and
scintillation on the point spread function (PSF) contrast achievable with
ground based adaptive optics (AO) are evaluated for different wavefront sensing
schemes. I show that a wavefront sensor (WFS) based upon the Zernike phase
contrast technique offers the best sensitivity to photon noise at all spatial
frequencies, while the Shack-Hartmann WFS is significantly less sensitive. In
AO systems performing wavefront sensing in the visible and scientific imaging
in the near-IR, the PSF contrast limit is set by the scintillation chromaticity
induced by Fresnel propagation through the atmosphere. On a 8m telescope, the
PSF contrast is then limited to 1e-4 to 1e-5 in the central arcsecond.
Wavefront sensing and scientific imaging should therefore be done at the same
wavelength, in which case, on bright sources, PSF contrasts between 1e-6 and
1e-7 can be achieved within 1 arcsecond on a 8m telescope in optical/near-IR.
The impact of atmospheric turbulence parameters (seeing, wind speed, turbulence
profile) on the PSF contrast is quantified. I show that a focal plane wavefront
sensing scheme offers unique advantages, and I discuss how to implement it.
Coronagraphic options are also briefly discussed.
