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Description
The definition, construction and realization of a reference system is one of the oldest and most fundamental tasks of astronomy. Currently, the ICRS (International Celestial Reference System), realized by the ICRF (International Celestial Reference Frame) or ICRF2, is the reference system adopted by the IAU. It is based on the very precise VLBI positions of a few hundred compact extragalactic radio sources. Despite its excellent precision, the ICRF is far from providing a reference system that is available for the entire sky and accessible to all observers. The ICRF has to be densified and extended to other wavelengths, mainly to the optical domain where the astronomical activities are more intense. For this reason, the IAU has recommended and encouraged works in this direction over the years. Many of them were developed to give sometimes good positions and proper motions, but they are very limited in magnitude, while others are extremely dense and deep in magnitude but have low accuracy, mainly for the proper motions. Nevertheless, all these contributions are very important because they are complementary. We present a homogeneous and precise optical astrometric catalog that extends the ICRF in the direction of 12 low-extinction windows of the Galactic bulge and provides at the same time a useful database for kinematic studies.
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