New algorithm for high-precision photometry in crowded star fields

Abstract

An increasing number of astronomical problems require accurate measurements of the brightness of stars situated in crowded fields. These include the search for extrasolar planets by the transit method, which looks for decreases in stellar brightness that happen when a planet passes in front of a star. In this case, the low probability that a planet will pass in front of its parent star implies that a large number of targets have to be monitored at the same time, hence the high degree of crowding. Other applications include dating the oldest stellar clusters and measuring the light curves of bright variable stars in external galaxies (the standard candles used to determine cosmological distances). When the fields are so crowded that the images of neighboring stars overlap, separating the stellar images requires a knowledge of the shape of point sources, the so-called point spread function (PSF). Most methods rely on images of sufficiently isolated stars to construct the PSF. However, in very crowded fields, no such isolated objects can generally be found and an accurate PSF cannot be constructed, which undermines the quality of the brightness measurements.