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J/MNRAS/430/2797    ICRF2 sources of the Rio survey          (Assafin+, 2013)

Rio survey of optical astrometric positions for 300 ICRF2 sources and the current optical/radio frame link status before Gaia. Assafin M., Vieira-Martins R., Andrei A.H., Camargo J.I.B., Da Silva Neto D.N. <Mon. Not. R. Astron. Soc., 430, 2797-2814 (2013)> =2013MNRAS.430.2797A
ADC_Keywords: Surveys ; Positional data ; Fundamental catalog Keywords: methods: observational - techniques: image processing - surveys - astrometry - reference systems - quasars: general Abstract: We obtained improved optical positions for 300 ICRF2 sources - the Rio survey. We compared the Rio survey with 10 other selected optical astrometric surveys and studied the link between the Hipparcos Catalogue Reference Frame (HCRF) and the International Celestial Reference Frame, Second Realization (ICRF2). We investigated the possible causes for the observed non-coincidence between the optical and ICRF2 positions. The Rio survey positions were referred to the second version of the United States Naval Observatory CCD Astrograph Catalog (UCAC2), currently the best tested HCRF densification. The sources are between -90°<δ<+30°. We used two telescopes with suitable diameters and focal lengths to properly link the observed ICRF2 sources with the UCAC2, using intermediate brightness stars. We certified the astrometry done with many statistical tests. The average 'optical minus ICRF2' offsets and respective standard deviations in (α, δ) were -3mas (41mas) and +4 (45mas). The Rio survey represents well the zero-point offset of the other surveys. The standard error of 3.5mas found for the HCRF/ICRF2 link indicates an error excess that can be originated by a non-coincidence between the observed optical/VLBI positions. We thus discussed the influence of the errors from the UCAC2. Then, we searched for correlations with the source morphology, represented by structure indices defined in the radio and in the optical domain. Finally, we studied how the position offsets could originate from the perturbation of the optical point spread function (PSF) of the source's core, by a second source of flux. We found an analytical relation that describes the resulting centroid shift, as a function of the atmospheric seeing, the brightness ratio and the relative distance between the two contributing flux sources. Two scenarios, modelled by this relation, are discussed: an extinction window in the dust torus nearby the core, and a Galactic star near the line of sight. Description: In our observations, we used two non-dedicated instruments, shared by the Brazilian astronomical community: the 0.6m diameter Bollen & Chivens Cassegrain telescope (F/13.5, f=8.1m) and the 1.6m diameter Perkin-Elmer Cassegrain telescope (F/10, f=16m), located at the LNA observing site, the Observatorio do Pico dos Dias, Brasopolis, Brazil (OPD/LNA) (IAU code 874; λ=+45:34:57, φ=-22:32:04, h=1870m). We added sources for observation later on, with the outcome of the ICRF-ext1 (Ma, 2001, 15th Working Meeting on European VLBI for Geodesy and Astrometry) and of the ICRF-ext2 (Fey et al., 2008, Cat. J/AJ/127/3587). Naturally, not all sources could be observed, due to prohibitive telescope time consuming. When the ICRF2 was finally released in 2009, the program at OPD/LNA had already finished. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file table2.dat 130 300 The 300 ICRF2 sources of the Rio survey
See also: I/323 : International Celestial Reference Frame 2, ICRF2 (Ma+, 2009) J/AJ/127/3587 : VLBI ICRF. II (Fey+, 2004) Byte-by-byte Description of file: table2.dat
Bytes Format Units Label Explanations
1- 8 A8 --- IERS IERS name (HHMM+DDd, B1950) 10- 11 I2 h RAh Right ascension (ICRS), at Epoch 13- 14 I2 min RAm Right ascension (ICRS), at Epoch 16- 22 F7.4 s RAs Right ascension (ICRS), at Epoch 24 A1 --- DE- Declination sign (ICRS), at Epoch 25- 26 I2 deg DEd Declination (ICRS), at Epoch 28- 29 I2 arcmin DEm Declination (ICRS), at Epoch 31- 36 F6.3 arcsec DEs Declination (ICRS), at Epoch 38- 41 F4.1 mag Vmag V magnitude 43- 51 F9.4 yr Epoch [1993/2009] Mean epoch of the position 53- 55 I3 mas sigX [3/193] photocentre measuring error along X associated to (RA, DE) 57- 59 I3 mas sigY [3/240] photocentre measuring error along Y associated to (RA, DE) 61- 62 I2 mas e_RAs Mean error along RA from (RA,DE) reduction 64- 66 I3 mas e_DEs Mean error along DE from (RA,DE) reduction 68- 70 I3 --- Nref [5/148] Number of reference stars 72- 75 I4 mas dRA Optical minus ICRF position offset in RAcosDE 77- 80 I4 mas dDE Optical minus ICRF position offset in Dec 82- 84 I3 mas sRA [0/161] Standard deviation of the contributing individual CCD positions along RA (1) 86- 88 I3 mas sDE [0/179] Standard deviation of the contributing individual CCD positions along DE (1) 90- 91 I2 --- No [1/40] Number of contributing individual positions 93- 96 F4.2 --- X ?=9.99 Structure index in X-band (∼8.3GHz) (2) 98-101 F4.2 --- S ?=9.99 Structure index in S-band (∼2.3GHz) (2) 103-106 F4.2 --- P ?=0.00 P=sqrt(K2+R2+N2) index (3) 108-112 F5.2 --- K ?=-1 Skewness (K index) (3) 114-118 F5.2 --- R ?=-1 Roundness (R index) (3) 120-124 F5.2 --- N ?=-1 Normalness (N index) (3) 126-128 F3.1 m Tel Telescope size 130 A1 --- --- [m]
Note (1): We compute the standard deviations (σαδ) when there are 3 or more individual CCD contributing positions. In the case of 2 contributing positions, we give the distance between the coordinates. In the case of only one position available, we don't estimate σ (code 00). Note (2): X and S indices (see Fey & Charlot 1997ApJS..111...95F, 2000, Cat. J/ApJS/128/17), extracted from the Bordeaux VLBI Image Database (BVID) and from the Radio Reference Frame Image Database (RRFID), related to the radio structure of the source core in the X and S bands. These indices are presented in Sect. 8.2 and discussed in detail in Sect. 8.3. Codes 9.99 for the X or S radio indices, and 0.00 for the optical indices, mean no index available. Note (3): Indices P, K, R, and N, with P=sqrt(K2+R2+N2), based on the LQAC2, related to the optical Point-Spread-Function (PSF) of the source. The K, R and N indices are respectively related to the skewness, roundness, and normalness of the PSF.
History: From electronic version of the journal
(End) Patricia Vannier [CDS] 11-Apr-2014
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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