Accurate Optical Positions of Extragalactic Radio Reference Frame Sources

Abstract

Optical positions on the 50 mas level in the Hipparcos system have been obtained for 327 extragalactic, radio reference frame sources for both hemispheres from a 2-step procedure. Positions of secondary reference stars in the 10 ¹ V ¹ 14 magnitude range were obtained from photographic plates taken at the Hamburg astrograph (Northern Hemisphere) and the US Naval Observatory astrograph at Black Birch (Southern Hemisphere) using Hipparcos stars for astrometric plate solutions. Positions of the optical counterparts of compact, extragalactic, radio reference frame sources were then obtained from CCD direct imaging at the KPNO and CTIO 0.9 m telescopes, using the secondary reference star positions and correcting for Ðeld distortions. Several previously unknown counterparts could be identiÐed, and several previously suspected identiÐcations turned out to be empty Ðelds. CCD images are available on the World Wide Web to serve as Ðnding charts. A comparison between the optical and radio positions reveals systematic o †sets of 10 to 40 mas per coordinate as a function of the observing run. Corrections for Galactic rotation and solar motion have been attempted because of a lack of individual proper motions for the secondary reference stars. Neither 0.9 m telescope is an astrometric instrument, and limitations are clearly seen in remaining systematic errors. Orientation angles between the radio system and our optical data are zero within an accuracy of 4 mas (standard error). A signiÐcant reduction of the systematic errors will be achieved with new wide-Ðeld CCD imaging currently been taken in parallel to new 0.9 m observations, to provide more secondary reference star positions with higher precision at a common epoch, which will allow extended modeling of the mapping properties of 0.9 m observations.

CCD observations of Phoebe, 9th satellite of Saturn

“…As demonstrated by (Charlot 1990;Fey & Charlot 1997), the position of an extragalactic radio-source and the position of its optical counterpart can be different. This difference is induced by the structure of the source, but because of the lower precision and accuracy of our optical observations of the counterparts, this does not affect our study (Zacharias et al 1999).…”

Near-infrared astrometry and photometry of Southern ICRF quasars

“…This represents a twofold process that has long been common practice in quasar optical astrometry, with traditionally wide field photographic plates, on the one hand, and narrow field CCD image, on the other (see e.g. Zacharias et al 1999). A similar densification process could be now obtained with the Wide Field Imager (WFI) mounted at the Cassegrain focus of the 2.2-m MPG/ESO telescope.…”

The celestial reference frame stability and apparent motions of the radio sources

“…In papers of Zacharias et al (1999), Assafin et al (2003) the catalogue of optical positions of 172 ICRF sources was composed. In the first paper a significant spacial difference between optical and radio quasar components was already pointed out.…”