Updating of B1950 radio star positions by means of J2000 calibrators


Roland Hering, Hans Georg Walter, Heidelberg



Radio star observations make use of extragalactic calibrators. The International Celestial Reference Frame (ICRF) provides updated calibrator positions referring to the coordinate frame of J2000.0. This fact stimulates the demand for a procedure that allows the relative observations of radio stars, ultimately reduced to B1950.0, to profit from the improved positions of the calibrators.

The transformation method discussed here goes back to the difference between the position vectors of the calibrator and radio star at the observation epoch of the latter, since that of the calibrator is normally not documented. By rotating the difference vector to the frame J2000.0 and adding to the vector of the calibrator, the updated radio star coordinates are obtained. Comparisons with results based on one of the standard transformation methods show offsets at the level of 5-10 milliarcseconds, which is a non-negligible quantity in modern astrometry. Other comparisons with the corresponding positions in the Hipparcos catalogue point into the right direction.

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Bibliographical details:

Roland Hering, Hans Georg Walter: Updating of B1950 radio star positions by means of J2000 calibrators. In: Peter Brosche, Wolfgang R. Dick, Oliver Schwarz, Roland Wielen (Eds.): The Message of the Angles - Astrometry from 1798 to 1998. Proceedings of the International Spring Meeting of the Astronomische Gesellschaft, Gotha, May 11-15, 1998. (Acta Historica Astronomiae; 3). Thun ; Frankfurt am Main : Deutsch, 1998, p. 198-200.