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I/275 The AC 2000.2 Catalogue (Urban+ 2001)
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AC 2000.2: The Astrographic Catalogue on the Hipparcos System
Urban S.E., Corbin T.E., Wycoff G.L., Hoeg E., Fabricius C., Makarov,V.V.
<Astron. J. 115, 1212 (1998)>
=1998AJ....115.1212U
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ADC_Keywords: Positional data ; Astrographic zones ; Surveys
Description:
The AC 2000.2 is a revised version of the 1997 release of the AC 2000
(Cat. <I/247>). It was decided that the availability of an improved
reference catalogue and the inclusion of photometry from the Tycho-2
catalogue would be sufficient to warrant a complete re-reduction of
the data and a new distribution of the catalogue. The AC 2000.2
catalog contains positions of 4,621,751 stars at the average epoch of
plate exposures for each star (average 1907).
Introduction:
The Astrographic Catalogue (AC) was an international effort designed
to photograph and measure the positions of all stars brighter than
magnitude 11.0. In total, some 4.6 million stars were observed, many
as faint as 13th magnitude. Users should realize that a few thousand
of the brightest stars are not included; their images on the source
plates were grossly overexposed and therefore measurements were often
not made. This project was started in the late 1800's, and the
positions that have been derived from the AC data are being used, in
combination with modern epoch positions, to determine accurate proper
motions.
The United States Naval Observatory has partnered with Copenhagen
University Observatory in completing the reductions of the
Astrographic Catalogue data (AC) to a consistent system. The resulting
catalog, called AC 2000.2, contains 4,621,751 stars covering the
entire sky at an average epoch of 1907, although the actual range in
epoch is from 1891 to 1950. The AC 2000.2 is a revised version of the
1997 release of the AC 2000 (Cat. <I/247>). AC 2000.2 positions are on
the Hipparcos Celestial Reference System (HCRS, coincident with
J2000.0) at the epochs of observation. Photometry is from the Tycho-2
Catalogue (Hoeg et al. 2000, Cat. <I/259>)) for the 2.5 million stars
in common; the remainder are derived from image diameter measures on
the photographic plates.
The new reductions were performed because of the availability of the
Hipparcos Catalogue (ESA 1997, Cat. <I/239>), which was not available
during the initial plate reductions of the AC 2000. (A conversion to
the Hipparcos system was performed as a last step in the AC 2000
reductions, however). In the original AC 2000, the reference catalogue
used was the ACRS (Corbin & Urban 1991, Cat. <I/171>). To improve the
astrometry in this second version, a new reference catalogue was
compiled utilizing 140 observed astrometric catalogues including
several, such as Tycho, which were unavailable during the original
reductions. Each of the input catalogues was reduced independently to
the Hipparcos system (HCRS) utilizing the Hipparcos data. The
individual catalogues were then combined into the 1999 version of the
Astrographic Catalog Reference Stars (ACRS_1999). The ACRS_1999 is not
available for public release since the data were subsequently used in
the Tycho-2 catalogue, hence it is obsolete. However, details of its
compilation can be found in the Introduction on the CD-ROM.
Each of the 22 zones making up the Astrographic Catalogue was reduced
independently using the ACRS_1999. Each was analyzed for tilt, radial
and tangential distortions, coma, magnitude equation, and
non-symmetric field distortions. The resulting data were then combined
into the final catalogue. Detailed information on the reduction
methodology and input data can be found in the Introduction (files in
the intro subdirectory) on the CD_ROM.
The data contain the positions (HCRS, coincident with J2000.0)
at mean epochs of observation, magnitudes in B and V from the
Tycho-2 catalogue where available -- else from image diameters --,
and accuracy estimates for each star. Cross identifications
with the Hipparcos and Tycho-2 catalogues are provided to
facilitate future work with these stars. Additionally,
the numbering from the original AC 2000 has been maintained. It should
be noted that there is not a strict one-to-one correspondence between
the two versions; the main reason is that some images now known
to be from high proper motion stars were not identified as such in
the 1997 version. However the numbering remains consistent. In other
words, the star numbered "1" in both versions refers to the same star.
Interested parties are encouraged to visit the AC web site at
http://ad.usno.navy.mil/ac/ac.html
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
content.txt 80 43 General information about the CD-ROM version
ac_p60.dat 93 258696 Stars with declinations +60 to +90 degrees (1)
ac_p50.dat 93 244782 Stars with declinations +50 to +59 59 59.99 (1)
ac_p40.dat 93 203384 Stars with declinations +40 to +49 59 59.99 (1)
ac_p30.dat 93 380756 Stars with declinations +30 to +39 59 59.99 (1)
ac_p20.dat 93 354027 Stars with declinations +20 to +29 59 59.99 (1)
ac_p10.dat 93 316295 Stars with declinations +10 to +19 59 59.99 (1)
ac_p00.dat 93 297604 Stars with declinations +00 to +09 59 59.99 (1)
ac_n00.dat 93 284059 Stars with declinations -00 to -09 59 59.99 (1)
ac_n10.dat 93 383610 Stars with declinations -10 to -19 59 59.99 (1)
ac_n20.dat 93 380218 Stars with declinations -20 to -29 59 59.99 (1)
ac_n30.dat 93 347379 Stars with declinations -30 to -39 59 59.99 (1)
ac_n40.dat 93 463841 Stars with declinations -40 to -49 59 59.99 (1)
ac_n50.dat 93 363806 Stars with declinations -50 to -59 59 59.99 (1)
ac_n60.dat 93 343294 Stars with declinations -60 to -90 degrees (1)
intro.tex 121 2512 Original Introduction (latex)
ps/* . 62 Detailed introduction in PostStcript and GIF.
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Note (1):
All of these files should be concatenated to give the
full AC 2000.2 catalog, split here into 14 files.
Within each file, stars are ordered by increasing right ascension.
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See also:
http://ad.usno.navy.mil/ac/ac.html : the AC web site
I/239 : The Hipparcos and Tycho Catalogues (ESA 1997)
I/246 : The ACT Reference Catalog (Urban+ 1997)
I/259 : The Tycho-2 Catalogue (Hog+ 2000)
Byte-by-byte Description of file: ac_p00.dat ac_p10.dat ac_p20.dat ac_p30.dat
Byte-by-byte Description of file: ac_p40.dat ac_p50.dat ac_p60.dat ac_n00.dat
Byte-by-byte Description of file: ac_n10.dat ac_n20.dat ac_n30.dat ac_n40.dat
Byte-by-byte Description of file: ac_n50.dat ac_n60.dat
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Bytes Format Units Label Explanations
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01-02 I2 h RAh R.A. (hours), HCRS, at epoch mEp
04-05 I2 min RAm R.A. (minutes), HCRS
07-12 F6.3 s RAs R.A. (seconds), HCRS
14-14 A1 --- DE- Decl. (sign), HCRS
15-16 I2 deg DEd Decl. (degrees), HCRS, at epoch mEp
18-19 I2 arcmin DEm Decl. (minutes), HCRS
21-25 F5.2 arcsec DEs Decl. (seconds), HCRS
27-31 F5.2 mag Bmag *Observed B magnitude
33-40 F8.3 yr mEp Mean epoch of position
42-43 I2 --- Num Number of images used for astrometry
44-49 F6.3 arcsec e_RAs *? Standard deviation of Right Ascension
50-55 F6.3 arcsec e_DEs *? Standard deviation of Declination
57-64 I8 --- ACnum USNO generated identifier
65-70 I6 --- HIP *? Hipparcos number
71-71 A1 --- m_HIP *Hipparcos component identifier
73-84 A12 --- TYC2 *Tycho-2 identifier
86-91 F6.3 mag Vmag *? V magnitude
92-92 I1 --- magflg *[1,3]? Magnitude warning flag
93-93 I1 --- verflg *[1]? Verification flag
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Notes on Bmag and Vmag:
The Bmag is either taken directly from the Tycho-2 Catalogue or is an
average of the computed magnitudes based on the measured image
diameters from the AC plates. To determine which is the case, one will
need to check the Vmag field. All stars with a non-blank Vmag field
contain Tycho-2 photometry in both the Bmag and Vmag fields. Stars
with a blank Vmag field contain B magnitudes from the measured image
diameters on the AC plates. In all cases, the Vmag data are from
Tycho-2. Note there are 20 stars where the Bmag is from Tycho-2 but
Tycho-2 did not contain a V magnitude. For these, a value of ".000" is
listed under Vmag and a value of "3" is set in the magflg field.
Notes on e_RAs and e_DEs:
These values are only available for stars with two or more images.
They are based on the scatter of the individual measures with respect
to the mean. As such, values can be very small or even 0.000.
Notes on HIP and m_HIP:
The Hipparcos numbers and component identifiers listed are generally
taken from the Tycho-2 main catalogue file. The vast majority of
Hipparcos stars are identified, but this cross reference is not 100%
complete.
Notes on TYC2:
The original Tycho-2 identifier consists of three fields, each
separated by a blank. The identifier given here is a concatenation of
these fields, with zeros replacing the blanks.
Notes on magflg:
This flag should caution users about possible problems in the photometry.
A value of "1" is given if the star has a Bmag value greater than 13.5
or a Vmag value greater than 12.5.
A value of "2" is given if the star was identified as a Tycho-2 star
but the Bmag is from the measured image diameters on the AC plates.
This is a result of either the Tycho-2 B_T magnitude is not available
or it is listed as fainter than 14.00. Note that no Vmag is given in
these cases. Where both flags "1" and "2" are set, only "2" is given.
A "3" is given if the star was identified as a Tycho-2 star, but there
is no Tycho-2 V_T magnitude. In these cases, the Tycho-2 V_T magnitude
is set to .000. There are 20 such instances. Only 1 star (AC~408635,
Tycho-2~107200084801) should have a "3" flag in combination with
another. For this star, the Tycho-2 B_T is 13.51 and no Tycho-2 V_T
magnitude exists (it is set to .000); The flag is set to "3" for this
star.
Number of records with magflg=1 is 211,796.
Number of records with magflg=2 is 32,489.
Number of records with magflg=3 is 20.
Notes on verflg:
This is either blank or "1". When set to "1", the star has not been
found in Hipparcos, Tycho-2, the overlapping AC plate or the Guide
Star Catalogue version 1.2. These records should be used with the
understanding that stars at these positions may not exist.
Number of records with verflg=1 is 199,039.
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Acknowledgments:
Astrometric reductions by the Astrometry Department
U.S. Naval Observatory
3450 Massachusetts Ave. NW
Washington, DC 20392-5420
e-mail: seu@pyxis.usno.navy.mil
Photometric reductions of the Tycho-2 stars by
Copenhagen University Observatory
Juliane Maries Vej 30
DK-2100 Copenhagen 0
Denmark
e-mail: tycho-2@astro.ku.dk
References:
The Tycho-2 Catalogue of the 2.5 Million Brightest Stars, 2000,
E. Hoeg, C. Fabricius, V.V. Makarov, S. Urban, T. Corbin,
G. Wycoff, U. Bastian, P. Schwekendiek, and A. Wicenec
A&A, 355, L27
The Hipparcos and Tycho Catalogues, 1997, ESA SP-1200
Astrographic Catalog Reference Stars, 1991,
T. Corbin and S. Urban, NASA, NSSDC 91-10
The AC 2000: The Astrographic Catalogue on the System
of the Hipparcos Catalogue, 1998, S.E. Urban, T.E. Corbin,
G.W. Wycoff, J.C. Martin, E.S. Jackson, M.I. Zacharias,
and D.M. Hall, AJ 115 1212
Usage Notice:
Any use of the data in publication must be accompanied by a citation
as to the original authors.
History:
The data supersede that from the 1997 version of "The AC 2000 Catalog."
The data are a copy of the CD-ROM "The AC 2000.2 Catalog" from
U.S. Naval Observatory and Copenhagen University Observatory,
October 2001.
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(End) Sean E. Urban [U.S. Naval Observatory] 22-Aug-2001