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The definition and the creation of the International Celestial Reference Frame (ICRF) was a joint cooperative effort of a sub-group of the International Astronomical Union (IAU) Working Group on Reference Frames. It was formed expressly for the purpose of creating the definitive catalogue of extragalactic radio source positions using the best data and methods available at the time the work was done. The ICRF was adopted by the International Astronomical Union as the fundamental celestial reference frame, replacing the FK5 optical frame as of 1998 January 1. The ICRF is the realization of the International Celestial Reference System (ICRS) at radio wavelengths. The Hipparcos Catalogue (1997), which includes all the FK5 stars, was astrometically aligned to the ICRF and currently provides the primary realization of ICRS at optical wavelengths. Gaia will create a much denser frame directly in the visible with an average of 20 sources per square degree outside the galactic plane, an increase by a factor of almost a thousand from the current ICRF. The image above depicts the sky distribution in galactic coordinates of the approximately 660 extragalactic sources that make up the International Celestial Reference Frame (ICRF). These are radio sources observed by long baseline interferometry and are primarily very faint in the visible. For more about the ICRF see the International Celestial Reference Frame web pages. Image courtesy of F. Mignard
[Published: 05/01/2004]
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