The Astronomical Journal, 129:2907–2913, 2005 June
# 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A.
ASTROMETRY OF INTERNATIONAL CELESTIAL REFERENCE FRAME SOURCES USING
THE SECOND US NAVAL OBSERVATORY CCD ASTROGRAPH CATALOG1
M. Assafin, P. T. Monken Gomes, and D. N. da Silva Neto,
Observatório do Valongo, Universidade Federal do Rio de Janeiro, Rua Ladeira Pedro Antonio 43,
CEP 20.080-090 Rio de Janeiro, RJ, Brazil; massaf@ov.ufrj.br
A. H. Andrei and R. Vieira Martins2
Grupo e Estudos em Astronomia, Observatório do Valongo, Universidade Federal do Rio de Janeiro, Rua Ladeira Pedro Antonio 43,
CEP 20.080-090 Rio de Janeiro, RJ, Brazil; and Observatório Nacional, Ministério da Ciência e Tecnologia,
Rua Gal. José Cristino 77, São Cristóvão, 20921-400 Rio de Janeiro, RJ, Brazil; oat1@ov.ufrj.br
and
3
J. I. B. Camargo, R. Teixeira, and P. Benevides-Soares
Instituto de Astronomia, Geofı́sica, e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão, 1226 Cidade Universitária,
CEP 05508-900 São Paulo, SP, Brazil; julio.camargo@obs.u-bordeaux1.fr
Receivved 2004 November 10; accepted 2005 February 19
ABSTRACT
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 Astrofı́sica, 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.
Key word: astrometry — catalogs — quasars: general — reference systems
1. INTRODUCTION
forward the optical representation of the ICRS. Proper motions
may then be derived for the first time for the fainter stars in these
sky regions. Such programs contribute directly to the determination of the actual alignment between the HCRS and the ICRS
through comparisons with the ICRF. In principle, because of the
faint magnitudes of the ICRF sources, large instruments are
needed for the task. However, large telescopes with time dedicated to astrometry are hard to find nowadays. On the other hand,
according to reports of the IAU Commission 8 working group on
‘‘The Future Development on Ground-Based Astrometry,’’ the
contribution of small- to medium-sized instruments to astronomy
is growing (Stavinschi & Kovalevsky 2001). Dedicated small
instruments are starting to play an important role in long-term
photometry programs, such as monitoring variable stars, microlensing events, etc. The same can be said about astrometry programs concerning Galilean satellites, Earth-grazing objects, the
semidiameter of the Sun, etc.
In this context, an ongoing long-term observational program
on the astrometry of southern ICRF sources is being carried out
at Laboratório Nacional de Astrofı́sica (LNA), Brazil, using two
Cassegrain telescopes of 0.6 and 1.6 m diameter equipped with
CCD detectors; these are not specific, dedicated astrometric
Since astrometric space missions will not produce scientific results until the next decade, ground-based astrometric observation
programs remain important. One fundamental task is the alignment maintenance of the Hipparcos Catalog Reference System
(HCRS; Rickman 2002) with respect to the International Celestial
Reference System (ICRS). Another objective is the densification
of the optical reference frame toward fainter magnitudes. Both
these tasks have an impact on astrometry at other wavelengths, on
astrometry with large and next-generation deep sky telescopes, and
even on the framework of space missions themselves.
In particular, programs for the optical observation of International Celestial Reference Frame (ICRF ) sources help push
1
Based on observations obtained at Laboratorio Nacional de Astrofı́sica
( LNA), Rua Estados Unidos, 154 Bairro das Nações, P.O. Box 21, 37500364
Itajubá, MG, Brazil.
2
Currently associated researcher at Institut de Mecanique Celeste et de
Calcul des Ephemerides, Observatoire de Paris, 77 Avenue Denfert Rochereau,
75014 Paris, France.
3
Currently visitor at Observatoire de Bordeaux, 2 Rue de l’Observatoire,
F-33270 Floirac, France.
2907
2908
ASSAFIN ET AL.
instruments. Here we present results of a pilot investigation for 31
ICRF sources, mostly south of the equator. The second US Naval
Observatory CCD Astrograph Catalog (UCAC2; Zacharias et al.
2004) is the primary reference frame for these observations. The
goal of the investigation is to access the astrometric performance
of these nonlarge telescopes in conjunction with UCAC2 in order
to establish the prospects for the success of the aforementioned
program. Observations of common fields made with the Valinhos
CCD Meridian Circle (VCMC; Instituto de Astronomia, Geofı́sica, e Ciências Atmosféricas, Universidade de São Paulo) contributed to the investigation of the UCAC2 performance. In x 2
we describe the observations and reduction procedures. In xx 3
and 4 we discuss the results and draw overall conclusions.
2. OBSERVATIONS AND REDUCTIONS
2.1. LNA Telescopes: Observations and Reductions
A total of 31 ICRF radio sources distributed in the range
90 þ20 were observed, mostly between 1997 and
2000. Observations were carried out with small- to medium-sized,
nondedicated telescopes: two fully automated Cassegrain telescopes with diameters of 0.6 m (f /13.5, f ¼ 8:1 m) and 1.6 m
(f /10, f ¼16 m), located at the LNA site (k ¼þ45 34 0 57 00 , ¼
22 32 0 04 00 , h ¼ 1870 m). The telescopes were equipped with
thin, back-illuminated CCD detectors (1024 pixel ; 1024 pixel;
pixels are of 24 m size), resulting in CCD fields of 5 0 ; 5 0 ,
1 pixel ¼ 0B3 for the 1.6 m telescope and 10 0 ; 10 0 , 1 pixel ¼ 0B6
for the 0.6 m telescope. The reference frame for the 0.6 m telescope CCD field reductions was furnished by the UCAC2 catalog
of compiled positions from observations made with the USNO
Twin Astrograph, profiting from a red filter in the 579–642 nm
spectral bandpass and corrected red lens, and many other early
epoch catalogs (Zacharias et al. 2004). For the 1.6 m telescope, the
reference frame came from the positions of fainter secondary stars
derived from the 0.6 m telescope CCD field reductions. All observations were made with the V filter (Johnson system) as a
compromise between the bandpass adopted for UCAC2 and the
bluer ICRF radio sources. Each field was observed at least three
times, with telescope shifts between exposures to avoid sampling
the same pixels on the star images. All exposures were guided.
Exposure times ranged from 90 up to 300 s, allowing efficient use
of telescope time and signal-to-noise ratios between 3 and 5. The
magnitude limit for the LNA site was estimated as V ¼ 21. The
same sources were observed at each telescope on time intervals
not longer than 2 yr, so as to avoid problems due to unknown
proper motions on the fainter noncataloged stars. An incidental
gap in the sample distribution at 3h 8h occurred.
Measurement and reductions followed the procedures described in detail in Assafin et al. (1997a, 1997b). All identified
objects in all CCD frames for each telescope had their (x, y)
measurements determined by bidimensional Gaussian fits. An
overlapping technique adapted from the method described in
Benevides-Soares & Teixeira (1992) was applied to transform
the (x, y) values of the observed fields into a single averaged (x, y)
instrumental frame for each telescope. This is done in a fashion
very similar to classical polynomial adjustments between measured and standard coordinates, but in this case the many (x, y)
values of the same field are adjusted against each other in an
iterative manner. After the (x, y) transformation, the ( , ) values
were then obtained by the usual reduction methods, using a
classical six-constant model to relate measured and standard
coordinates with regard to the reference catalog. In the case of
the 1.6 m telescope, the reference frame came from the 0.6 m
telescope field reductions, formed by positions of secondary
Vol. 129
field stars located within 5 0 ; 5 0 of the source position. In the case
of the 0.6 m telescope, the reference catalog used was UCAC2.
No proper motions were applied here because the time interval
between observations and the UCAC2 average epoch is less
than 2 yr. Magnitudes were computed on the basis of the fitting
of image profiles and relate to the magnitude system of UCAC2
by means of a simple logarithmic relation and the adjustment of
a zero point—they are not photometrically derived from flux
measurements and should be used with care.
Figure 1 shows the (x, y) measurement error estimates from
the Gaussian fits as a function of the magnitude (UCAC2 instrumental magnitude system) for each telescope. The behavior
of the (x, y) Gaussian measurements with magnitude is as expected, with errors increasing toward the faint end. For a given
magnitude value, the better performance of the 1.6 m telescope
with regard to the 0.6 m telescope is explained by the larger
aperture and favorable pixel scale (greater diameter and twice
the focal distance). In the case of the 0.6 m telescope, about 65%
of the stars have their (x, y) measurement errors below 15 mas
(magnitudes up to ~16). This includes most UCAC2 reference
stars but not many secondary stars. Toward the faintest limits,
say up to the 19th magnitude, the error is greater than 40 mas on
that telescope, but then only one observed source was that faint.
Besides, statistics are less reliable there, since this faint magnitude range represents only 6% of the total 0.6 m telescope
measurements. In the case of the 1.6 m telescope, most stars lay
below 15 mas, reaching 20 mas and above only in extreme cases
for magnitudes over 20 mag, where no sources were observed.
Finally, the small error peaks in x and y around the 17th magnitude on the 0.6 m telescope graphics are the contribution of
pixel-undersampled, fainter stars from a small group of short
90 s CCD exposures. Similar peaks around the 11th magnitude
are caused in turn by pixel saturation on brighter stars from a
small group of deep 300 s CCD exposures.
Considering reference stars alone, all other computed error estimates such as (O C) residuals, standard errors, etc., display
no dependencies with magnitude or location on the CCD frames
for both telescopes. On average, 30 reference stars on the 0.6 m
telescope and 28 stars on the 1.6 m CCD fields were used, with
typically less than 10% outliers being excluded in the reductions.
Averages for the errors commented in the following are listed on
Table 1.
While (x, y) Gaussian errors refer to stellar image scales,
(O C) residuals from the x, y transformation of individual
(x, y) measurements contain astronomical, atmospherical, and instrumental errors at much larger areas on the CCD frames. The
formal standard deviations of the residuals of the x, y transformations were 18 and 15 mas for the 0.6 and 1.6 m telescopes,
respectively, on both coordinates. A random increase of residuals toward fainter magnitudes occurs in the same fashion as
with Gaussian errors. The similar residuals that were found for
the (x, y) transformations reflect the star distributions with magnitude for each telescope and the respective Gaussian (x, y) error
distributions shown on Figure 1. This indicates that no further
significant degradation occurred because of other atmospherical
or instrumental effects over the entire CCD fields. It also ensures
the good intrinsic astrometric quality of the 0.6 m telescope
frames within the range of its magnitude limits.
The standard deviations for the unweighted ( , ) reductions
were 31 and 30 mas for the 0.6 and 1.6 m telescopes, respectively,
for both coordinates. They agree with the expected errors of the
reference catalog for the 15th magnitude—the average of the
UCAC2 reference stars used in the reductions. The standard errors for the right ascension and declination of the sources were
No. 6, 2005
ASTROMETRY OF ICRF SOURCES
2909
Fig. 1.—The (x, y) measurement error estimates from Gaussian fits with respect to magnitude (UCAC2 instrumental magnitude system). For the 1.6 m telescope,
each point on average represents 55 stars over 2413 measured objects, and for the 0.6 m telescope each point on average represents 77 stars over a total of 4383 objects.
computed directly from the variance-covariance matrix in the
( , ) least-square adjustments following Eichhorn & Williams
(1963). They were, on average, 9 mas for both coordinates and
telescopes. In all, the error estimates for the (x, y) measurements
and ( , ) reductions were similar for both instruments. Thus, we
must consider a possible random error propagation in magnitude
from the Gaussian fits and from the (x, y) transformations, affecting the positions of the fainter 0.6 m telescope stars, which serve
as a reference frame for the 1.6 m field reductions. Random error
propagation from the 0.6 m telescope ( , ) solutions themselves
should also be expected (see more comments on this issue in x 3).
our V-band CCD frames, since they were referred to UCAC2,
a catalog from observations on the 579–643 bandpass between
V and R. That is, we needed a set of independent V-band star
positions similar to UCAC2 in epoch, magnitude range, and
position precision. For that, we observed 17 common fields
taken with the V filter with VCMC (k ¼ þ46 58 0 03 00 , ¼
23 00 0 06 00 , h ¼ 850 m). The observations with VCMC were
carried out in the period 1998–2000 in drift-scanning mode,
resulting in fields of 130 extension in declination and 30 minutes in
right ascension. On average, 10 fields were observed per source,
with an overlapping technique being used to reduce positions. All
the thousand stars identified in these fields had positions referred
to the Tycho-2 catalog, the primary reference frame of UCAC2
itself. Only those portions of the sky corresponding to 100 around
the source positions were used. Visual magnitudes were also calculated, giving values ranging from 8 to 15 for the VCMC stars
2.2. UCAC2 and its Magnitude Bandpass System
Applied to V-Band Observations
We needed to investigate whether unaccounted magnitude/
color effects might have eventually affected the reductions of
TABLE 1
Error Estimates: (x, y) Measurements and ( , ) Reductions
0.6 m Telescope Errors
( , )
Reductions
(mas)
1.6 m Telescope Errors
( , )
Reductions
(mas)
(x, y)
Measurements
(mas)
(x, y)
Measurements
(mas)
Statistics
N
E
E
Ex
Ey
N
E
E
Ex
Ey
Average ........................
Dispersion ....................
30
30
7
31
8
9
6
9
6
18
6
17
5
28
31
8
29
7
9
7
9
6
15
7
14
6
Note.—Error estimates for the (x, y) measurements and ( , ) reductions for the sample of 31 ICRF sources. N stands for the average number of reference stars; Ex
and Ey are the error estimates of (x, y) measurements for the entire CCD frames [standard deviations of the residuals from the (x, y) overlapping transformation
adjustments; see text for explanation]; and are the formal standard deviations from the (O C ) residuals of reference stars from the ( , ) reductions; and E and
E are the formal standard errors of the source positions from the variance-covariance matrix of ( , ) reductions.
2910
ASSAFIN ET AL.
Vol. 129
dependent fields (17) are considered. In all, the analysis above
indicates that UCAC2 behaves similarly to any V catalog, not
introducing any systematic errors in our reductions regarding
magnitude or color, on both equatorial coordinates.
3. RESULTS
Fig. 2.—Position offsets (nonabsolute), VCMC UCAC2, against V magnitudes (Valinhos instrumental system) for right ascension. The error bars refer
to the formal standard deviations within each bin (average of 84 stars per bin). A
similar magnitude equation was found by Dominicci et al. (1999) and confirmed
by Camargo et al. (2003).
used. A detailed description of the instrument and astrometric
procedures is given in Viateau et al. (1999).
As a starting point, the VCMC positions of all 17 common fields were directly compared against their corresponding
UCAC2-band positions given in UCAC2. For the 594 common
stars, the VCMC UCAC2 position differences were, on average, +77 mas (69 mas) and 16 (74 mas) for right ascension and
declination, respectively. (Formal standard deviations are given in
parentheses.) The right ascension average difference is significant
and is due to a magnitude equation. Plots of position offsets with
respect to magnitude gave a magnitude equation for right ascension, with (nonabsolute) right ascension position offsets systematically increasing toward fainter magnitudes. This feature
is displayed in Figure 2. It plots the right ascension position
offsets (nonabsolute) as VCMC UCAC2 against V magnitudes
(Valinhos instrumental system). Error bars stand for the formal
standard deviations within each bin. An analogous magnitude
equation was found and described by Dominicci et al. (1999)
and confirmed by Camargo et al. (2003) when comparing position
offsets between the VCMC and the Bordeaux CCD Meridian
Circle. Thus, the magnitude equation can be safely ascribed to the
VCMC exclusively. No dependencies on the position offsets were
found with regard to color (V R), even after eliminating the
magnitude equation from the VCMC right ascensions. We must
then conclude that no magnitude/color effect arises from using
UCAC2 field stars. This conclusion is further supported by a direct comparison against the positions of the VLBI sources. In this
case, we reduced the 0.6 m CCD fields with the available VCMC
positions. Only those CCD frames with at least 10 VCMC reference stars available were reduced. This yielded the positions of
six ICRF sources. Average values and errors for the VCMC
radio position offsets were +89 29 mas (71 mas) and 31
29 mas (72 mas) for right ascension and declination, respectively. (Values in parentheses refer to standard deviations.) The
VCMC-based optical radio right ascension offset equals the
previously found VCMC UCAC2 average difference in right
ascension, implying no mismatch between the UCAC2 and
radio frames. The direct VCMC UCAC2 average declination difference also matches the VCMC-based optical radio
offset within the combined errors. Besides, the direct average
declination difference (16 mas) significance is found to be low
when the standard deviation (74 mas) and the number of in-
Table 2 list the results for all 31 ICRF sources observed at the
1.6 and 0.6 m LNA telescopes. Optical radio position offsets
refer to reductions made with the 0.6 and 1.6 m telescope frames.
Offsets in parentheses were omitted from the statistics given at
the bottom of the table because they refer to reductions with five
or less reference stars only, which is much less than the average
number of 30 for both telescopes. The average values are listed
together with the respective formal standard errors (in parentheses) and standard deviations. Epochs and magnitudes refer to
the mean 0.6 and 1.6 m telescope respective values. The number
of reference stars used in the reductions is also given. No observations and therefore no positions were obtained with the 1.6 m
telescope for sources 1253055 and 1921293 at the time of
this publication. The bright, optically extended source1228+126
presents a jetlike feature extending up to about 1500 northwest
from the center in all 0.6 m telescope CCD images. Tentative
reductions with manual removal of pixels visually associated to
the feature were attempted. Two results with and without pixel
removal are given on Table 2. In spite of our efforts, no reliable
results were achieved for this source, since the values are much
larger than the typical optical radio offsets obtained.
3.1. Astrometric Performance of the Telescopes
The optical radio positions offset averages and standard
errors for the 0.6 m telescope were +4 8 mas (41 mas) and +1
8 mas (42 mas) for right ascension and declination, respectively.
(Values in parentheses refer to standard deviations.) For the 1.6 m
telescope, the offsets were 12 9 mas (45 mas) and +8 9 mas
(46 mas). Since the dominating source of systematic error (i.e.,
offsets) comes from the primary reference stars’ zero points on
right ascension and declination in each field, it is bound to be the
same for both telescopes because the 1.6 m telescope data is derived from the 0.6 m astrometric solution. At any rate, the averages are statistically null, indicating no misaligment between
UCAC2 and the ICRF at the 10 mas level. Statistics for the 0.6 m
results refer to 24 optical ICRF position offsets, with 24 other
offsets for the 1.6 m telescope comparisons.
The results are in agreement with the UCAC2 position errors
at the observation epoch. As given by the standard deviations,
the 1.6 m telescope positions are consistent with the ICRF within
45 mas and those for the 0.6 m telescope are consistent within
41 mas. The similarity of the standard deviations for such different telescopes supports the strategy of observing secondary
stars with the 0.6 m telescope and binding them to the UCAC2
stars in order to have them provide a fainter, numerous, and
accurate representation of UCAC2 in the small CCD frames from
the 1.6 m telescope. This enables us to bridge the magnitude gap
and extend the program to the faintest end of the optical counterparts of the ICRF sources, while being bound to the UCAC2/
Hipparcos reference system. It is also verified that no larger CCD
nor mosaics of frames are required, since Table 2 shows equal
O C standard deviations ( , ) for both the faint nonUCAC2 stars at the 1.6 m telescope and the UCAC2 stars at the
0.6 m telescope. On the other hand, since the gain of precision on
the centroid determination (Ex, Ey) from the 0.6 to the 1.6 m
telescope is modest for the brighter ICRF sources optical counterparts, the determination of their position solely by the 0.6 m
No. 6, 2005
ASTROMETRY OF ICRF SOURCES
2911
TABLE 2
Results for 31 ICRF Sources for the 1.6 and 0.6 m LNA Telescopes
1.6 m Telescope
0.6 m Telescope
UCAC2/0.6 m Based Opt Rad
(mas)
UCAC2 Based Opt Rad
(mas)
ICRF Source
Epoch
Magnitude
cos
N
cos
N
0047579 .....................................
0237+040 ......................................
0919260 .....................................
0920397 .....................................
1032199 .....................................
1034293 .....................................
1129580......................................
1144379......................................
1228+126 ......................................
1228+126 ......................................
1253055 .....................................
1320446 .....................................
1329665 .....................................
1354+195 ......................................
1355416 .....................................
1435218 .....................................
1510089 .....................................
1511100......................................
1514+197 ......................................
1514241 .....................................
1538+149 ......................................
1656+053 ......................................
1921293 .....................................
1925610 .....................................
2037253 .....................................
2052474 .....................................
2128123 .....................................
2204540 .....................................
2251+158 ......................................
2255282 .....................................
2326477 .....................................
2328+107 ......................................
Average .........................................
Standard deviation ........................
95.6493
98.9096
98.8233
98.0671
97.8877
99.3370
99.3411
97.7918
99.5014
99.5014
99.5014
98.9151
98.4904
97.4712
97.7493
99.3383
99.3685
97.4397
99.3657
95.8767
97.4438
97.0644
97.6575
97.9986
97.9959
98.2041
96.5904
96.5644
98.3123
96.1945
96.1259
98.3123
17.3
17.9
17.9
18.0
19.0
18.1
18.4
17.8
15.2
15.6
15.6
19.1
18.8
16.4
15.2
18.5
16.9
18.3
18.7
15.4
17.1
16.7
16.4
19.7
19.3
17.5
15.3
18.1
16.0
16.6
16.0
18.2
+25
15
+42
2
+67
11
40
36
...
...
...
+50
60
(77)
+34
(131)
20
90
10
(+9)
54
25
...
45
47
51
54
(+90)
40
+82
+50
26
12 (9)
45
+48
+6
26
+30
+0
7
+22
+28
...
...
...
+88
+75
(141)
+26
(16)
35
48
+17
(+3)
+46
34
...
+10
66
+43
111
(179)
9
+42
+48
29
+8 (9)
46
8
6
24
37
11
22
113
24
...
...
...
62
93
(5)
52
(5)
30
18
12
(5)
15
44
...
18
33
21
10
(4)
8
7
7
9
18
...
+78
+49
+26
+1
+58
78
(206)
(359)
+47
+67
...
(69)
+27
...
30
37
20
+32
15
17
+2
...
13
16
77
...
+20
18
+05
+28
+4 (8)
41
+14
...
21
+5
+52
+03
+63
+18
(+95)
(+188)
+19
+79
...
(67)
08
...
33
+2
8
35
+23
+14
41
...
44
+25
47
...
25
+21
+57
113
+1 (8)
42
6
...
43
37
10
09
172
22
(5)
(5)
12
57
...
(5)
24
...
23
21
8
24
13
22
132
...
22
9
15
...
12
6
9
16
Notes.—Results for 31 ICRF sources for the 1.6 and 0.6 m LNA telescopes. Optical ICRF radio position offsets (Opt Rad) refer to reductions made with the
UCAC2 catalog (UCAC2 based, 0.6 m telescope) and a secondary catalog (UCAC2/0.6 m based, 1.6 m telescope). The average values are listed together with the respective
formal standard errors (in parentheses) and standard deviations. Epochs (1900 plus table value) and magnitudes refer to the mean 0.6 and 1.6 m values. N stands for the number
of reference stars used in the reductions. No observations and therefore no positions were obtained at the 1.6 m telescope for sources 1253055 and 1921293 at the time of
this publication. Two results for source 1228+126 are given (0.6 m telescope), with (first result) and without (second result) taking into account a jetlike feature in the images.
telescope is validated, which considerably speeds up the realization of the 1.6 m telescope program.
The samples investigated here encompass well-imaged sources
for both telescopes. For the 10% of the sources successfully observed only at the 1.6 m telescope (see Table 2), the optical
radio offsets (included in the statistics) remained within expected
values and did not affect the comparison of results between the
telescopes. Likewise, the standard deviation on the 1.6 m telescope optical ICRF positions of sources dimmer than 19 mag
is 43 mas. Accordingly, the position offset distributions of both
telescopes are point-to-point similar within 97% or better, applying nonparametric Spearman rank-correlation statistics. This
means that the UCAC2 reference frame was still preserved in the
positions derived from the 0.6 m telescope reductions and passed
forward to the 1.6 m telescope CCD frames.
The overall behavior of the optical radio position offsets
with respect to right ascension and declination is shown by the
vector map of Figure 3, which refers to the 1.6 m telescope
results. The 0.6 m telescope offsets give a similar plot. Arrows
point to the relative optical position. For better visualization,
right ascensions of two sources between 0h and 3h are plotted
from 24h on. Although in general the offset sizes vary, it seems
from Figure 3 that for small regions in the sky represented by
adjacent vectors similar directions prevail. We generated a random distribution of offset vectors and took the angles between
vectors of adjacent sources. To check whether the 0.6 and 1.6 m
local distributions really did not behave randomly, we compared their respective angle averages with those expected of a
random distribution. After that, we applied the test of equal
means using Student’s t-statistics. We found for both instruments a 90% or better probability that the vector directions for
adjacent sources are indeed not aleatory. Here an interpretation
of this result is unclear because of the small sample at hand. A
deeper investigation of the issue of optical radio position offset
distributions, based on a broader observed sample in the context
of examining dense astrometric catalogs such as the USNO-B1
2912
ASSAFIN ET AL.
Fig. 3.—Vector map of optical radio position offsets with respect to right
ascension and declination for the 1.6 m telescope results. The 0.6 m telescope
offsets give a similar plot. Arrows point to the relative optical position. For a
better visualization, the right ascensions of two sources between 0h and 3h are
plotted from 24h on.
(Monet et al. 2003) and the Two Micron All Sky Survey (Cutri
et al. 2003), is given in da Silva Neto et al. (2005).
We also investigated the presence of differential color refraction, since the sources are bluer than the average reference
stars in the CCD fields. Indeed, the size and distribution of the
optical radio declination offsets versus declination (observations were made close to the meridian) were not incompatible
with differential color refraction with a V filter (Kovalevsky
1997). Linear fits of these offsets against declination were then
tried for both telescopes. Nominally, the obtained correction
reduced the standard deviation of the optical ICRF offsets to
37 mas. However, when the errors on the determination of linearfit coefficients are added to the error budget, no net gain is verified
with respect to the adherence to the ICRF. The conclusion of the
LNA program will bring a much larger number of observations,
which shall clarify the issue.
Finally, no correlation between the optical ICRF radio
positions with source brightness is found.
3.2. Comparison with Independent Instrument/Catalog
Sets of Source Positions
To further test the astrometric performance of our telescopes
and UCAC2, we compared our results with other independent
Vol. 129
instrument/catalog sets of source positions. For that, the list of
327 sources from Zacharias et al. (1999) was an interesting
choice. In that case, short-focus astrographs and long-focus reflectors were used on plate/CCD observations. The positions
were directly referred to the HCRS from the astrograph fields. To
account for the unknown proper motions of the secondary stars
linking short- and long-focus fields, solar motion and Galactic rotation were considered. The paper listed positions with and without these kinematic corrections. Another list considered is given
by Assafin et al. (2003), which is independent with regard to telescope (0.9 m telescope at Cerro Tololo Inter-American Observatory [CTIO]) and reduction procedures but which shares the same
reference catalog, UCAC2.
The comparison between these lists of optical positions is
shown on Table 3. Only sources without optical radio offsets
marked in parentheses in Table 2 were considered. Table 3 displays differences, namely, this work minus other lists. Zacharias
[C] and Zacharias [N] refer to the optical positions of 20 common sources in Zacharias et al. (1999) with [C] and without [N]
kinematic corrections, respectively. CTIO refers to the optical
positions of eight common sources in Assafin et al. (2003). Averages and formal standard deviations for the position differences
are furnished. The adherence to the ICRF of our UCAC2-based
LNA positions has already been shown by the averages at the
beginning of x 3.1. It is further supported by the statistically null
differences between the common positions of ourselves (LNA)
and CTIO. However, the difference between the common LNA
and Zacharias et al. (1999) positions is statistically significant in
declination. Such a difference does not originate from the positions here obtained, since for the 20 common sources our offsets,
LNA ICRF, are (for the 1.6 m telescope observations) 11
10 mas (46 mas) and 02 10 mas (43 mas) for right ascension
and declination, respectively. This is interpreted as indicating that
the accuracy of the UCAC2 positions supersedes previous catalogs and specific lists, such as the set of radio source positions by
Zacharias et al. (1999) in this case.
4. SUMMARY AND CONCLUSIONS
We present results of a pilot investigation on the astrometric
performance of small- and medium-sized telescopes and the
UCAC2 catalog. For this, 31 ICRF sources, mostly south of the
equator, were observed during 1997–2000 at the 1.6 and 0.6 m
Cassegrain telescopes at LNA, Brazil. The source positions
were referred to UCAC2. Observations were made in the V band
in a compromise between the 579–643 nm bandpass of UCAC2
and the bluer ICRF sources. An investigation was made to certify
the UCAC2-band positions of UCAC2 as a good reference frame
for the CCD fields observed in the V band. For this, UCAC2 was
compared with an independent set of star positions of similar
epoch, magnitude range, and precision in the V band. This set of
TABLE 3
Comparison of Results with Other Optical Position Lists
Us Zacharias [C]
(mas)
Us Zacharias [ N]
(mas)
Us CTIO
(mas)
Statistics
cos
cos
cos
Average ..................................
Standard deviation .................
+3 (18)
80
29 (10)
45
1 (15)
68
53 (10)
45
3 (18)
53
15 (21)
60
Notes.—Comparison of results with other optical position lists: this work minus another list. Zacharias [C] and Zacharias [N] refer to the
optical positions of 20 common sources in Zacharias et al. (1999) with [C] and without [N] corrections, respectively, for solar motion and
Galactic rotation. CTIO refers to the optical positions of eight common sources in Assafin et al. (2003; see discussion in x 3.2).
No. 6, 2005
ASTROMETRY OF ICRF SOURCES
positions was obtained with the Valinhos CCD Meridian Circle,
Brazil. A magnitude equation was found in the study but associated entirely to the VCMC positions, a result also reported
independently by Dominicci et al. (1999) and confirmed by
Camargo et al. (2003). No other magnitude or color effects were
found that would otherwise have limited the use of UCAC2 in
the reductions.
The final average values and errors of the optical radio
position offsets for 24 sources using the 0.6 m telescope were
+4 8 mas (41 mas) and +1 8 mas (42 mas) for right ascension and declination, respectively. (Values in parentheses
refer to standard deviation.) For the 1.6 m telescope, 24 other
sources gave offsets of 12 9 mas (45 mas) and +8 9 mas
(46 mas). Although, as expected, the stars’ (x, y) measurement
errors increase toward fainter magnitudes, it was verified that
the sets of positions are still statistically consistent with the
UCAC2 frame being properly passed from the 0.6 to the 1.6 m
telescope positions. A linear adjustment was tried on the optical
ICRF declination offsets against declination in favor of a possible
differential color refraction for a V filter (Kovalevsky 1997); this
nominally reduced the standard deviations of the optical ICRF
offsets in declination. However, when the errors on the coefficients
of the linear fit are considered, there is no net gain. The correction
was therefore not applied and the effect will be further investigated
when a larger sample becomes available. Other external comparisons with independent instrument/catalog sets of high-precision
2913
source positions, namely, the lists of optical positions given by
Zacharias et al. (1999) and Assafin et al. (2003), show results
consistent with the respective errors.
In all, using the UCAC2 as the primary reference frame one
can expect that individual sources positions will be on the HCRS
within at least 40 mas. An ongoing long-term program investigating the optical astrometry of southern ICRF sources is being
carried out at both telescopes at LNA. Because of the limited number of available sources, a detailed investigation of zonal tendencies and their origins and of differential color refraction shall only
be attempted after the conclusion of the entire program. Nevertheless, the results so far obtained indicate the great potential of small- to medium-sized Cassegrain telescopes for accurate
astrometric work, particularly on the astrometry of the optical
counterpart of extragalactic radio sources.
M. A. acknowledges the Fundação de Amparo à Pesquisa do
Estado do Rio de Janeiro (grant E-26/170.686/2004) for their
support. D. N. S. N. is grateful to the Brazilian Conselho Nacional
de Desenvolvimento Cientı́fico e Tecnológico (grant 303950/
2003-0). A. H. A. thanks the Centre National del la Recherche
Scientifique for contract QAF183803. R. V. M. is thankful to the
Brazilian Coordenação de Aperfeiçoamento de Pessoal de Nivel
Superior (grant 0449/04-0).
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