Content of the Tycho-2 CD-ROM

readme.txt : file with a short description of the Tycho-2 Catalogue

format.txt : file with formats of the four catalogue files,
also described in the Guide.

data/ : directory with the catalogue files.
The main catalogue (catalog.dat, 528 301 904 bytes),
the two much shorter supplements (suppl_1.dat and suppl_2.dat),
and the index file (index.dat).

docs/ : directory with documentation.
Guide to the Tycho-2 Catalogue (guide.pdf and .ps),
the two figures from the Guide as high quality postscript files,
and two papers submitted to Astron. Astrophys. about the
catalogue (aa2000a.pdf and .ps) and about its construction and 
verification (aa2000b.pdf and .ps).

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/


                The Tycho-2 Catalogue

    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


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:

 1. For ALL entries: The observational Tycho-2 results (i.e. position 
    and one or two magnitudes), at the epoch of observation.
 2. 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.