Content of the Tycho-2 CD-ROM readme.txt : file with a short description of the Tycho-2 Catalogue format.txt : file with formats of the four catalogue files, also described in the Guide. data/ : directory with the catalogue files. The main catalogue (catalog.dat, 528 301 904 bytes), the two much shorter supplements (suppl_1.dat and suppl_2.dat), and the index file (index.dat). docs/ : directory with documentation. Guide to the Tycho-2 Catalogue (guide.pdf and .ps), the two figures from the Guide as high quality postscript files, and two papers submitted to Astron. Astrophys. about the catalogue (aa2000a.pdf and .ps) and about its construction and verification (aa2000b.pdf and .ps). You may want to consult the Tycho-2 home page: http://www.astro.ku.dk/~erik/Tycho-2 for updates and for further information. A perl program for extracting data from the catalogue is available from: http://archive.eso.org/ASTROM/ The Tycho-2 Catalogue The principal characteristics of the Tycho-2 Catalogue are summarized below. By means of proper motions the positions are transferred to the year 2000.0, the epoch of the catalogue. The median values of internal standard errors are given. ------------------------------------------------------------ Mean satellite observation epoch ~J1991.5 Epoch of the Tycho-2 Catalogue J2000.0 Reference system ICRS coincidence with ICRS (1) +-0.6 mas deviation from inertial (1) +-0.25 mas/yr Number of entries 2,539,913 Astrometric standard errors (2) V_T < 9 mag 7 mas all stars, positions 60 mas all stars, proper motions 2.5 mas/yr Photometric std. errors (3) on V_T V_T < 9 mag 0.013 mag all stars 0.10 mag Star density b= 0 deg 150 stars/sq.deg. b= +-30 deg 50 stars/sq.deg. b= +-90 deg 25 stars/sq.deg. Completeness to 90 per cent V ~ 11.5 mag Completeness to 99 per cent V ~ 11.0 mag Number of Tycho observations ~300 10^6 ------------------------------------------------------------ Note (1): about all 3 axes Note (2): ratio of external to internal standard errors is ~1.0 for positions and for proper motions. Systematic errors are less than 1 mas and 0.5 mas/yr Note (3): ratio of photometric external to internal standard errors at V_T > 9 mag is below 1.5 The Tycho-2 Catalogue is an astrometric reference catalogue containing positions and proper motions as well as two-colour photometric data for the 2.5 million brightest stars in the sky. Components of double stars with separations down to 0.8 arcsec are included. The Tycho-2 positions and magnitudes are based on precisely the same observations as the Tycho-1 Catalogue (ESA SP-1200, 1997) collected by the star mapper of the ESA Hipparcos satellite, but Tycho-2 is much bigger and slightly more precise, owing to a more advanced reduction technique. Proper motions precise to about 2.5 mas/yr are given as derived from a comparison with the Astrographic Catalogue (AC) and 143 other ground-based astrometric catalogues, all reduced to the Hipparcos celestial coordinate system. For only about 100,000 stars, no proper motion could be derived. Tycho-2 supersedes Tycho-1, and the ACT and TRC catalogues based on Tycho-1. The main catalogue, catalog.dat, gives: 1. For ALL entries: The observational Tycho-2 results (i.e. position and one or two magnitudes), at the epoch of observation. 2. For most entries (96%): Proper motions (at epoch 2000) derived using other catalogues (mainly AC) and the corresponding mean positions at epoch J2000, as well as mean epochs and error estimates. For double stars resolved in Tycho-2, but where the proper motion is for the photocentre, the same mean values are given to both components. When no proper motion has been derived, no mean position for epoch J2000 is given. Supplement-1, suppl_1.dat, gives: This file contains stars missing in Tycho-2, but found in HIP or Tycho-1. The supplement-1 only includes Tycho-1 stars of good quality and therefore the quality 9 (very poor) stars and probable side-lobes were not included. Supplement-2, suppl_2.dat, gives: This file contains the probable Tycho-1 sidelobes (quality 1-8) which were excluded from supplement-1. These stars are either false entries in Tycho-1 or stars where the result is heavily disturbed by nearby, much brighter stars. The quality 9 stars were also excluded from supplement-2. A look-up table, index.dat, is provided for the main catalogue and supplement-1. This file can be used to quickly find all stars located in a given field.