QSOs photometric identification for astrometric reduction of CCD images

International audienceBecause of their important distances, Quasi-Stellar Objects (QSOs) seem point-like sources with no apparent proper motion. This is the reason why QSOs are quasi-ideal objects for linking a CCD image to another one or for linking directly a CCD image to the OCRF and with the forthcoming 'GAIA reference Frame'. One of the difficulties of astrometric reduction using QSOs as reference points is to distinguish QSOs from stars in CCD images. We present in this poster three distinct QSOs iden- tification methods based only on the photometric characteristics of QSOs and we study the advantages and disadvantages of each one from an astrometric point of view