The Hipparcos and Tycho Catalogues are the primary products of the European Space Agency’s astrometric mission, Hipparcos. The satellite, which operated for four years, returned high quality scientific data from November 1989 to March 1993.

The Tycho-2 Catalogue is an astrometric reference catalog containing positions and proper motions as well as two-colour photometric data for the 2.5 million brightest stars in the sky. The Tycho-2 positions and magnitudes are based on precisely the same observations as the original Tycho Catalogue (hereafter Tycho-1; see CDS Cat. I/239) 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. Components of double stars with separations down to 0.8 arcsec are included. Proper motions precise to about 2.5 mas/yr are given as derived from a comparison with the Astrographic Catalogue and 143 other ground-based astrometric catalogs, all reduced to the Hipparcos celestial coordinate system. Tycho-2 supersedes in most applications Tycho-1, as well as the ACT (CDS Cat. I/246) and the TRC (CDS Cat. I/250) catalogs based on Tycho-1.

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 catalog. 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

Source: http://heasarc.nasa.gov/W3Browse/all/tycho2.html

For more information on the original catalog, please consult the Tycho-2 home page at http://www.astro.ku.dk/~erik/Tycho-2