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 In 1996 the now superseded release 1 of these data on the FK5 system was published, see Zacharias et al, AJ 112, 2336.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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