StarFinder is a code designed to analyze Adaptive Optics images of very crowded fields. A typical AO observation has a well sampled and complex shaped PSF, showing a sharp peak, one or more fragmented diffraction rings and an extended irregular halo. The approach followed in StarFinder is to analyze the stellar field with a digital image of the PSF, without any analytic approximation

Under the assumptions of isoplanatism and well-sampling, StarFinder models the observed stellar field as a superposition of shifted scaled replica of the PSF lying on a smooth background due to faint undetected stars, possible faint diffuse objects and noise.

The procedure derives first a PSF digital template from the brightest isolated field stars; then a catalogue of presumed objects is formed, searching for the relative intensity maxima in the frame.

In the following step the images of the suspected stars are analyzed in order of decreasing luminosity; each suspected object is accepted on the basis of its correlation coefficient with the PSF template; the relative astrometry and photometry of the source are determined by means of a fit, taking into account the contribution of the local non-uniform background and of the already detected stars.

At the end of the analysis it is possible to remove the contribution of all the stars detected up to this point and perform a new search for previously lost objects (e.g. secondary components of close binaries)

StarFinder is not a general purpose algorithm for object recognition: it should be intended as a tool to obtain high precision astrometry and photometry in adequately sampled high resolution images of crowded stellar fields. In practice it can be applied not only to high-Strehl AO observations, but also to low-Strehl images and HST fields, provided the sampling is satisfactory.

Much more intriguing and difficult to solve is the case of a field with space variant PSF, due to anisoplanatic effects in AO observations. In general the analysis of an anisoplanatic field requires the knowledge of the local PSF. The extension of StarFinder to the space variant case is in progress.Preliminary results have been presented at the ESO/SPIE meeting Astronomical Telescopes and Instrumentation 2000,(see reference 3).

StarFinder Code References:

1. E.Diolaiti, O.Bendinelli, D.Bonaccini, L.Close, D. Currie, and G.Parmeggiani, "Analysis of isoplanatic high resolution stellar fields by the StarFinder code",
accepted for publication in "Astron. & Astrophys. Sup. Ser. ", 2000.
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2. E. Diolaiti, O.Bendinelli, D. Bonaccini, L.Close, D. Currie and G.Parmeggiani, "STARFINDER: a Code to Analyse Isoplanatic High-Resolution Stellar Fields ",
in "The Messenger", No. 100, p.23—27, 2000.
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3. E.Diolaiti, O.Bendinelli, D.Bonaccini, L.M.Close, D.G.Currie and G.Parmeggiani, "StarFinder:an IDL GUI based code to analyze crowded fields with isoplanatic correcting PSF fitting",
in "Proceeedings of Adaptive Optical Systems Technology", P.L.Wizinowich, ed., Proc. of SPIE, Vol.4007,pp. 879—888, 2000.
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4. E. Diolaiti, O.Bendinelli, D. Bonaccini, L. M. Close, D.G. Currie and G.Parmeggiani, "StarFinder: a code for crowded stellar fields ",
in "Proceedings of ADASS IX", D. Crabtree, N. Manset and C. Veillet, eds., ASP Conference Series, San Francisco, 2000. In press.
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5. E.Diolaiti, O.Bendinelli, D.Bonaccini, G.Parmeggiani and F.Rigaut, "An algorithm for crowded stellar fields analysis ",
in "Proceedings of the ESO/OSA Topical Meeting on Astronomy with Adaptive Optics", D.Bonaccini, ed., pp.175—184, ESO, Garching, 1999.
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