|
Description
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. The Tycho-2 positions
and magnitudes are based on precisely the same observations as the
original Tycho Catalogue (hereafter Tycho-1)
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
catalogues, all reduced to the Hipparcos celestial coordinate system.
Tycho-2 supersedes in most applications Tycho-1, as well as the ACT
and TRC catalogues based on Tycho-1.
Supplement-1 lists stars from the Hipparcos and Tycho-1 Catalogues
which are not in Tycho-2. Supplement-2 lists 1146 Tycho-1 stars which
are probably either false or heavily disturbed.
For more information, please consult the Tycho-2 home page:
http://www.astro.ku.dk/~erik/Tycho-2
Catalogue Characteristics:
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
|
