The Tycho-2 Catalogue:
Short Description


This readme file gives a short description of the Tycho-2 Catalogue. A more detailed description is given in the file (or guide.pdf) in the docs directory. There you will also find two papers submitted to Astron. Astrophys. about the catalogue (aa2000a.pdf and .ps) and about its construction and verification (aa2000b.pdf and .ps). The formats of the data files are described in the files "format.htm" and "format.txt" as well as in the Guide. The principal characteristics of the catalogue are summarized in a table. The catalogue files are in the data directory. You may want to consult the Tycho-2 home page:   for updates and for further information. A perl program for extracting data from the catalogue is available from

Introduction to the Catalogue

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:

Supplement-2, suppl_2.dat, gives:

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.