Accurate Astrometry and Photometry of Saturated and Coronagraphic Point Spread Functions

Abstract

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 (AO) 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, such as coronagraphy with a nontransmissive 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. We show 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 AO instruments and should be considered for future extreme AO coronagraph instruments and in high-contrast imaging space observatories.