Twin Astrographic Catalog Version 2 (TAC 2.0)

last update: NZ 991025, now version 2.0 complete

  1. Introduction
  2. Files
  3. Data Example
  4. Data Format
  5. Remarks

1) Introduction

The Twin Astrographic Catalog (TAC) is an astrometric, photographic catalog covering most of the sky between +90 and -18 degrees declination to an average limiting magnitude of about V=11 and B=12. In 1996 the now superseded release 1 of these data on the FK5 system was published, see Zacharias et al, AJ 112, 2336. This release 2 is based on the same 4912 plates as release 1 which were taken with the U.S. Naval Observatory Twin Astrograph (blue and yellow lens) between 1977 and 1986. The Hipparcos Catalogue has been used for a new plate-by-plate rigorous reduction. A significant improvement over the release 1 version of the data was achieved. With an average precision of 48 to 120 milliarcsecond (mas) per coordinate, depending on magnitude, and a higher star density than the Tycho-1 Catalogue, the TAC is a significant catalog for proper motion determination.

This release 2 of the TAC contains high quality positions of 705,099 stars, supplemented by photographic photometry (B and V, on Tycho system) for most of these stars. It supersedes the release 1 data of the TAC. The release 1 data were organized in separate files for each declination zone, sorted by right ascension. This release 2 is a single file covering the entire area, sorted strictly by declination. No proper motions are given in this release 2. The data provide high precision positions on the International Celestial Reference System (ICRS), corresponding to an equinox of J2000, at a mean epoch of 1982. The TAC 2.0 data are to be used in combination with other astrometric catalogs to get precise, current epoch positions.

For details see the paper "TAC on the Hipparcos System" by Zacharias and Zacharias, AJ 118, 2503 (Nov) 1999 issue of the Astronomical Journal. For proper use of the data, please read the following sections carefully, in particular the remarks section.

2) Files

The following files are given in this release:

  tac.html     = this file, introduction, data format explanation
  tac_2.dat.gz = the data, compressed 20 MB, (ASCII, 54 MB uncompressed)
  check_tac.f  = Fortran program to check the catalog data   (optional)
  check_tac.log= output log file of that check program       (optional)
  tac_paper.ps = Post-Script file of the TAC release 2 paper (optional)

3) Data Example

The first and last 10 lines of the main data file are given below. The columns are explained in the following section.

ISN            RA       SPD eRA eDC ni epoch Vtac Btac  eV  eB y b  Vty  Bty
 22224  677606866 257450658 148 130  2 80216  934  943   0   0 1 1  934  978
 21792  663127957 257455613  28 410  2 82165 1120 1187   0   0 1 1 1089 1246
 21352  644143309 257457318  99 168  2 82165 1134 1197   0   0 1 1    0    0
 22398  684178766 257494759 163 121  2 80216 1137 1139   0   0 1 1    0    0
 21314  642895464 257502121 114 137  2 82165 1119 1170   0   0 1 1    0    0
 22173  676393510 257522761  54  50  2 80216 1149 1195   0   0 1 1    0    0
 21040  628901766 257532216 171 172  1 82231    0 1107   0   0 0 1 1120 1165
 20935  629653835 257535065   7 296  2 82231 1081 1107   0   0 1 1 1096 1157
 20812  625545913 257537515  43  41  2 82231 1028 1013   0   0 1 1 1029 1070
 19289  582193900 257538274  39 160  2 82288 1040 1117   0   0 1 1 1030 1081

800184 1289649394 646001629  21  78  4 85181  941 1062   1   8 2 2  957 1085
800268  775680221 646006594  76  83  4 85316 1032 1114   3   2 2 2 1039 1088
800232  725136731 646207757  58  30  6 85260 1068 1139   3   4 3 3 1066 1105
800186 1280912964 646353013  31  54  4 85181 1013 1082   2   4 2 2 1018 1067
800207  527693424 646449831  23  39  4 85032  910  947  12   3 2 2  909  936
800230  638737597 646515745  44  50  4 85151 1050 1123   5   3 2 2 1052 1125
800205  482733793 646558828  54  47  4 85032 1050 1124   5   1 2 2 1047 1110
800197  254454917 646668448  39  31  4 85179  919 1084   3   3 2 2  939 1135
800191   28434453 646793594  42  54  4 85181 1042 1126   8   1 2 2 1048 1123
800189 1149129641 647186670  54  36  4 85182  969 1022   1   3 2 2  971 1000

4) Data Format

column      unit  explanation
----------------------------------------------------------------------------
ISN               internal star number
RA       mas      right ascension, ICRS (J2000), epoch of observation
SPD      mas      south pole distance = declination + 90 degree, ICRS ...
eRA      mas      formal standard error in RA * cos DC
eDC      mas      formal standard error in declination
ni                number of images used for the position
epoch  1/1000 yr  mean epoch of observation (-1900)
Vtac   1/100 mag  photographic V magnitude from TAC data
Btac   1/100 mag  photographic B magnitude from TAC data
eV     1/100 mag  formal error on photographic V mag
eB     1/100 mag  formal error on photographic B mag
y                 number of yellow plate images used for Vtac
b                 number of blue plate images used for Btac
Vty    1/100 mag  V magnitude from the Tycho Catalogue
Bty    1/100 mag  B magnitude from the Tycho Catalogue

5) Remarks

The catalog data can be read in free format: there are no blank entries, and there is at least one blank between all columns on all lines. All entries are positive integer numbers.

The catalog contains 705099 stars on 705099 lines. The internal star number (ISN) is a running number, similar to the strategy adopted for the Hipparcos Catalogue. All TAC ISN are in the range from 1 to 800342. Each star has a unique ISN, however not all ISN in that range are used. A star in this catalog should be referred to by its ISN preceded by the acronym of the catalog (TAC) or/and its position. There will be no confusion with the numbering in the release 1 TAC, because there a scheme by declination zones was adopted for the catalog data and the nomenclature.

The catalog includes single image stars and is sorted by declination. Stars with a formal error larger than 500 mas in at least one coordinate have been excluded and are not present in the catalog. The precision of the TAC catalog is a function of magnitude, see paper, ranging from 48 to 120 mas for stars in the 8 to 12 magnitude range.

Note, instead of the declination, the south polar distance (SPD) is given, which equals the declination plus 90 degrees and is always positive. The weighted mean epochs for the RA and DC coordinates can be different, however usually only on the order of 1/100 years, and only the mean for both coordinates is given in the catalog.

Errors given for the positions are formal, internal errors, obtained from the weighted scatter of individual images (see paper). The formal errors on the single-image stars are larger than zero, including the assumed measuring precision. For individual stars they might be grossly wrong. Errors given for the photometry are zero when only one image was available to estimate the magnitude. The errors of the photographic magnitudes in general are underestimated due to the strong correlation with the Tycho photometry. The user is encouraged, whenever possible, to use the Tycho magnitudes, attached at the end of the TAC data line, which are believed to be more precise and more reliable than the photographic magnitudes.

Photographic magnitudes for bright stars ($\le$ 6$^{m}$) should not be used in any case, because they can be wrong by several magnitudes due to over- exposure effects. Note, an absence of a Tycho magnitude attached to a star in the TAC could still mean the star is a Tycho star. Tycho magnitudes have been attached for most, not all, Tycho stars in the TAC, based on photometric criteria.

Images of close double stars were merged and handled as a single star and given a single ISN. If the scatter of those images exceed 500 mas, the star is not in the catalog. Thus this catalog is likely to miss many double stars, with their individual position measures being doubtful anyway due to the low resolution of the photographic images from our short focal length instrument. The final catalog has been matched with itself and no pairs within a box of 3 arcseconds were found.

6) Contact

Please direct scientific questions about the TAC to nz@usno.navy.mil

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