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 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:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 130 300 The 300 ICRF2 sources of the Rio survey
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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
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Bytes Format Units Label Explanations
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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]
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Note (1): We compute the standard deviations (σα,σδ)
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 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.
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 11-Apr-2014