Astrometry of International Celestial Reference Frame Sources Using the Second US Naval Observatory CCD Astrograph Catalog

Abstract

We present results of a pilot investigation on the astrometry ofInternational Celestial Reference Frame ( ICRF) sources using small- tomedium-sized telescopes and the second US Naval Observatory CCDAstrograph Catalog (UCAC2). For this purpose, 31 ICRF sources wereobserved, mostly south of the equator, during 1997 - 2000. We used theautomated 0.6 and 1.6 m Cassegrain telescopes equipped with CCDdetectors located at Laboratorio Nacional de Astrofisica, Brazil. Thesource positions were referred to UCAC2, with fainter 0.6 m telescopestars serving as a reference frame to the reductions of the 1.6 mtelescope CCD fields. Observations were made in the V band in acompromise between the 579 - 643 nm bandpass ( between V and R) of UCAC2and the bluer ICRF sources. To ensure that UCAC2, with its magnitudebandpass system, is a reliable reference catalog for our V-band CCDframe reductions, we have also compared it against an independent set ofstar positions with similar characteristics obtained in the V band withthe Valinhos CCD Meridian Circle, Brazil. Average values and errors forthe optical - radio position offsets using the 0.6 m telescope were + 4+/- 8 mas ( 41 mas) and + 1 +/- 8 mas ( 42 mas) for right ascension anddeclination, respectively. ( Parenthetical values refer to standarddeviation, i. e., to the typical error of a single measurement given thequantity of sources.) For the 1.6 m telescope, offsets were - 12 +/- 9mas (45 mas) and + 8 +/- 9 mas (46 mas). An expected random error thatincreases with magnitude and affects the positions of the fainter 0.6 mtelescope secondary stars is verified. No systematic errors were foundwithin the attained position precision, including differential colorrefraction. External comparisons with independent telescope/catalog setsof precise source positions were also made, showing consistent resultswithin the respective errors.