An Improved Technique for the Photometry and Astrometry of Faint Companions

We propose a new approach to differential astrometry and photometry of faint companions in adaptive optics images. It is based on a prewhitening matched filter, also referred to in the literature as the Hotelling observer. We focus on cases where the signal of the companion is located within the bright halo of the parent star. Using real adaptive optics data from the 3 m Shane telescope at the Lick Observatory, we compare the performance of the Hotelling algorithm with other estimation algorithms currently used for the same problem. The real single-star data are used to generate artificial binary objects with a range of magnitude ratios. In most cases, the Hotelling observer gives significantly lower astrometric and photometric errors. In the case of high Strehl ratio (SR) data (SR approximate to 0.5), the differential photometry of a binary star with Delta m = 4.5 and a separation of 0.6 is better than 0.1 mag; a factor of 2 lower than the other algorithms considered.