  Accurate astrometry and photometry of saturated and coronagraphic point
spread functions (PSFs) are fundamental to both ground- and space-based high
contrast imaging projects. For ground-based adaptive optics imaging,
differential atmospheric refraction and flexure introduce a small drift of the
PSF with time, and seeing and sky transmission variations modify the PSF flux
distribution. For space-based imaging, vibrations, thermal fluctuations and
pointing jitters can modify the PSF core position and flux. These effects need
to be corrected to properly combine the images and obtain optimal
signal-to-noise ratios, accurate relative astrometry and photometry of detected
objects as well as precise detection limits. Usually, one can easily correct
for these effects by using the PSF core, but this is impossible when high
dynamic range observing techniques are used, like coronagrahy with a
non-transmissive occulting mask, or if the stellar PSF core is saturated. We
present a new technique that can solve these issues by using off-axis satellite
PSFs produced by a periodic amplitude or phase mask conjugated to a pupil
plane. It will be shown that these satellite PSFs track precisely the PSF
position, its Strehl ratio and its intensity and can thus be used to register
and to flux normalize the PSF. A laboratory experiment is also presented to
validate the theory. This approach can be easily implemented in existing
adaptive optics instruments and should be considered for future extreme
adaptive optics coronagraph instruments and in high-contrast imaging space
observatories.
