A method to analyze adaptive optics images of binary stars

¹ Dipartimento di Astronomia, Universitá di Bologna, Via Zamboni 33, 40050 Bologna, Italy
² European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85746 Garching bei Muenchen, Germany
³ Osservatorio Astronomico di Bologna, Via Zamboni 33, 40050 Bologna, Italy

Send offprint request to: O. Bendinelli

Received: 15 December 1997
Accepted: 2 October 1998

Abstract

We present a fast and robust ad-hoc method to analyze a set of short exposure images of a close binary star taken by the adaptive optics system of the ESO 3.6 m telescope. At first each image is fitted, assuming as a radially symmetrical approximation of its "instantaneous” PSF, the superposition of a central Gaussian and of a Moffat torus. Owing to the coral variability and asymmetry of the PSF the fit gives a poor estimate of the component's luminosity ratio; nevertheless it reproduces satisfactorily the global structure of the PSF and above all gives the accurate sub-pixel positions of the two stars. This allows to obtain, by the shift-and-add procedure, the mean set image from which the final parameters of the two stars and the mean PSF during the observing run are derived. The results from a set of AO images of the close binary system τ Canis Maioris are consistent with those previously obtained by one of us using the iterative blind deconvolution. Extensive simulations give evidence that the magnitude difference and the separation of the components can be estimated within about 0.05 mag and 0005 respectively when the secondary star image does not overlap a bright artifact of the AO system.