The Tycho-2 Catalogue:
Principal characteristics


Mean satellite observation epoch~J1991.5
Epoch of the Tycho-2 CatalogueJ2000.0
Reference SystemICRS
 coincidence with ICRS1±0.6 mas
 deviation from inertial1±0.25 mas
Number of entries2 539 913
Astrometric standard errors2
 VT < 9 mag7 mas
 all stars, positions60 mas
 all stars, proper motions2.5 mas/yr
Photometric std. errors3 on VT
 VT < 9 mag0.013 mag
 all stars0.10 mag
Star density
 b= 0o150 stars deg-2
 b=±30o50 stars deg-2
 b=±90o25 stars deg-2
Completeness to ~ 90 per centV ~ 11.5
Completeness to ~ 99 per centV ~ 11.0
Number of Tycho observations~ 300 million
Principal characteristics of the Tycho-2 Catalogue. By means of the proper motions, the positions are transferred to the year 2000.0, the epoch of the catalogue. The median values of the internal standard errors are given.

1about all three axes
2ratio of external to internal standard errors is ~ 1.0 for positions and for proper motions.Systematic errors are < 1 mas and < 0.5 mas/yr
3ratio of photometric external to internal standard errors at VT > 9 is below 1.5.