| Mean satellite observation epoch | ~J1991.5 |
| Epoch of the Tycho-2 Catalogue | J2000.0 |
| Reference System | ICRS |
| | coincidence with ICRS1 | ±0.6 mas |
| | deviation from inertial1 | ±0.25 mas |
| Number of entries | 2 539 913 |
| Astrometric standard errors2 |
| | VT < 9 mag | 7 mas |
| | all stars, positions | 60 mas |
| | all stars, proper motions | 2.5 mas/yr |
| Photometric std. errors3 on VT |
| | VT < 9 mag | 0.013 mag |
| | all stars | 0.10 mag |
| Star density |
| | b= 0o | 150 stars deg-2 |
| | b=±30o | 50 stars deg-2 |
| | b=±90o | 25 stars deg-2 |
| Completeness to ~ 90 per cent | V ~ 11.5 |
| Completeness to ~ 99 per cent | V ~ 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. |
