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>Astrometry of International Celestial Reference Frame Sources Using the Second US Naval Observatory CCD Astrograph Catalog*
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Astrometry of International Celestial Reference Frame Sources Using the Second US Naval Observatory CCD Astrograph Catalog*
We present results of a pilot investigation on the astrometry of International Celestial Reference Frame (ICRF) sources using small- to medium-sized telescopes and the second US Naval Observatory CCD Astrograph Catalog (UCAC2). For this purpose, 31 ICRF sources were observed, mostly south of the equator, during 1997–2000. We used the automated 0.6 and 1.6 m Cassegrain telescopes equipped with CCD detectors located at Laboratório Nacional de Astrofísica, Brazil. The source positions were referred to UCAC2, with fainter 0.6 m telescope stars serving as a reference frame to the reductions of the 1.6 m telescope CCD fields. Observations were made in the V band in a compromise between the 579–643 nm bandpass (between V and R) of UCAC2 and the bluer ICRF sources. To ensure that UCAC2, with its magnitude bandpass system, is a reliable reference catalog for our V-band CCD frame reductions, we have also compared it against an independent set of star positions with similar characteristics obtained in the V band with the Valinhos CCD Meridian Circle, Brazil. Average values and errors for the 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 and declination, respectively. (Parenthetical values refer to standard deviation, i.e., to the typical error of a single measurement given the quantity of sources.) For the 1.6 m telescope, offsets were -12 ± 9 mas (45 mas) and +8 ± 9 mas (46 mas). An expected random error that increases with magnitude and affects the positions of the fainter 0.6 m telescope secondary stars is verified. No systematic errors were found within the attained position precision, including differential color refraction. External comparisons with independent telescope/catalog sets of precise source positions were also made, showing consistent results within the respective errors.
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