Accurate Astrometry and Photometry of Saturated and Coronagraphic Point Spread Functions

Accurate astrometry and photometry of saturated and coronagraphic pointspread functions (PSFs) are fundamental to both ground- and space-based highcontrast imaging projects. For ground-based adaptive optics imaging,differential atmospheric refraction and flexure introduce a small drift of thePSF with time, and seeing and sky transmission variations modify the PSF fluxdistribution. For space-based imaging, vibrations, thermal fluctuations andpointing jitters can modify the PSF core position and flux. These effects needto be corrected to properly combine the images and obtain optimalsignal-to-noise ratios, accurate relative astrometry and photometry of detectedobjects as well as precise detection limits. Usually, one can easily correctfor these effects by using the PSF core, but this is impossible when highdynamic range observing techniques are used, like coronagrahy with anon-transmissive occulting mask, or if the stellar PSF core is saturated. Wepresent a new technique that can solve these issues by using off-axis satellitePSFs produced by a periodic amplitude or phase mask conjugated to a pupilplane. It will be shown that these satellite PSFs track precisely the PSFposition, its Strehl ratio and its intensity and can thus be used to registerand to flux normalize the PSF. A laboratory experiment is also presented tovalidate the theory. This approach can be easily implemented in existingadaptive optics instruments and should be considered for future extremeadaptive optics coronagraph instruments and in high-contrast imaging spaceobservatories.Comment: 25 pages, 6 figures, accepted for publication in Ap