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J/AJ/125/2728       Optical positions of ICRF sources        (Assafin+, 2003)

Optical positions of ICRF sources using UCAC reference stars. Assafin M., Zacharias N., Rafferty T.J., Zacharias M.I., Da Silva Neto D.N., Andrei A.H., Vieira Martins R. <Astron. J. 125, 2728 (2003)> =2003AJ....125.2728A
ADC_Keywords: Positional data ; QSOs Keywords: astrometry - catalogs - quasars: general - reference systems Abstract: New optical positions on the 30mas precision level have been obtained for 172 extragalactic International Celestial Reference Frame (ICRF) sources mainly in the range -30°≤DE≤+25°. Results are presented from a pilot investigation including four Cerro Tololo (CTIO) 0.9m runs (1999-2001). Reference stars in the R∼10-16.5mag used. Systematic errors have been investigated, and a field distortion pattern based on the residuals has been removed. The errors of the fainter stars in the CTIO data were assessed by evaluating an auxiliary set of CCD observations of common ICRF sources, taken at the 1.60m Cassegrain telescope of the Laboratorio Nacional de Astrofisica, Brazil. A significant improvement in the optical positions was achieved over a previous determination of source positions. The mean optical positions are compared with the ICRF radio positions. The overall optical minus radio offsets are -6 and -15mas for right ascension and declination, respectively. The formal internal error of these mean offsets is ∼2.3mas. his indicates a possible systematic error in the UCAC declinations of 10 to 15mas. Both the optical counterpart observations and the optical reference stars are observed about 9 yr after the Hipparcos mean epoch, and our results set an upper limit for a possible Hipparcos system rotation with respect to the International Celestial Reference System for the z-axis of about 0.7mas/yr. Description: Reference stars on the Hipparcos frame were provided by an almost all-sky catalog of prerelease UCAC2 positions. No proper motions are available; however, the UCAC data and our deep CCD images of the ICRF counterparts have been taken at almost the same epoch (around 1999 to 2001). For details about the project and observations the reader is referred to the UCAC1 and UCAC2 papers (Zacharias et al., 2000, Cat. I/268 and 2003, in prep.). File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file table4.dat 101 172 Optical positions for 172 ICRF sources from CTIO CCD observations
See also: I/251 : VLBI International Celestial Reference Frame (ICRF) (Ma+, 1997) I/239 : The Hipparcos and Tycho Catalogues (ESA 1997) I/268 : UCAC1 Catalogue (Zacharias+ 2000) I/294 : The UCAC2 Bright Star Supplement (Urban+, 2004) Byte-by-byte Description of file: table4.dat
Bytes Format Units Label Explanations
1- 8 A8 --- ICRF The ICRF source name 10- 11 I2 h RAh Hour of optical right ascension (ICRF) 13- 14 I2 min RAm Minute of optical right ascension (ICRF) 16- 22 F7.4 s RAs Second of optical right ascension (ICRF) 24 A1 --- DE- Sign of the optical declination (ICRF) 25- 26 I2 deg DEd Degree of optical declination (ICRF) 28- 29 I2 arcmin DEm Arcminute of optical declination (ICRF) 31- 36 F6.3 arcsec DEs Arcsecond of optical declination (ICRF) 38- 42 F5.3 arcsec e_RAs ? Standard deviation of the mean in RA (1) 44- 48 F5.3 arcsec e_DEs ? Standard deviation of the mean in DE (1) 50- 51 I2 --- Num Number of CCD observations 53- 59 F7.2 yr Epoch Mean epoch of the observations 61- 65 F5.2 mag Rmag The UCAC R-band magnitude 67- 70 F4.2 mag e_Rmag ? Standard deviation of the mean in Rmag (1) 72- 77 F6.3 arcsec DRAcosDE Right ascension optical-radio offset 79- 84 F6.3 arcsec DDE Declination optical-radio offset 86- 90 F5.3 arcsec sigmaRA Mean right ascension error of the "plate solution" 92- 96 F5.3 arcsec sigmaDE Mean declination error of the "plate solution" 98-100 A3 --- Comm Source CCD image peculiarity comments (2)
Note (1): Derived from the scatter of the individual values of each source. Note (2): Comments: ext = Extended object; nbo = Blended image or nearby object(s); cro = Cosmic ray near object (on at least one field); chp = Cold or hot pixels near source image. All cro, nbo, and chp source images were individually handled in an iterative fitting procedure for obtaining their (x,y) Gaussian centroids.
History: From electronic version of the journal
(End) Greg Schwarz [AAS], Patricia Bauer [CDS] 16-Jun-2003
The document above follows the rules of the Standard Description for Astronomical Catalogues.From this documentation it is possible to generate f77 program to load files into arrays or line by line

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