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The Tycho-2 Catalogue:
Short Description
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Abstract
This readme file gives a short description of the
Tycho-2 Catalogue. A more detailed description is given in the file guide.ps (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: http://www.astro.ku.dk/~erik/Tycho-2
for updates and for further information. A perl program for extracting
data from the catalogue is available from http://archive.eso.org/ASTROM/.
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:
- For ALL entries: The observational Tycho-2 results (i.e. position and
one or two magnitudes), at the epoch of observation.
- 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:
- This file contains stars missing in Tycho-2, but found in HIP or Tycho-1.
The supplement-1 only includes Tycho-1 stars of good quality and
therefore the quality 9 (very poor) stars and probable side-lobes were
not included.
Supplement-2, suppl_2.dat, gives:
- This file contains the probable Tycho-1 sidelobes (quality 1-8) which
were excluded from supplement-1. These stars are either false entries in
Tycho-1 or stars where the result is heavily disturbed by nearby, much
brighter stars. The quality 9 stars were also excluded from supplement-2.
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.