The Tycho-2 Catalogue is an

astronomical catalogue An astronomical catalogue is a list or tabulation of astronomical objects, typically grouped together because they share a common type, Galaxy morphological classification, morphology, origin, means of detection, or method of discovery. The olde ...

of more than 2.5 million of the brightest

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

Catalogue

The

astrometric Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way. History ...

reference catalogue contain positions,

proper motion Proper motion is the astrometric measure of changes in the apparent places of stars or other celestial objects as they move relative to the center of mass of the Solar System. It is measured relative to the distant stars or a stable referenc ...

s, and two-color photometric data for 2,539,913 of the brightest stars in the Milky Way. Components of double stars with separations down to 0.8

arcsecond A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

s are included. The catalogue is 99% complete to magnitudes of V~11.0 and 90% complete to V~11.5. (, Table 1) The Tycho-2 positions and magnitudes are based on the observations collected by the star mapper of the

European Space Agency The European Space Agency (ESA) is a 23-member International organization, international organization devoted to space exploration. With its headquarters in Paris and a staff of around 2,547 people globally as of 2023, ESA was founded in 1975 ...

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of ...

satellite. They are the same observations used to compile the Tycho-1 Catalogue (ESA SP-1200, 1997). However, Tycho-2 is much larger and a bit more precise, because a more advanced reduction technique was used. The U.S. Naval Observatory (USNO) first compiled the ACT Reference Catalog, (Astrographic Catalogue / Tycho) containing nearly one million stars, by combining the Astrographic Catalogue (AC 2000) with the Tycho-1 Catalogue; the large

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided b ...

span between the two catalogues improved the accuracy of proper motions by about an order of magnitude. Tycho-2 now supersedes the ACT. Proper motions precise to about 2.5 milliarcseconds per year are given as derived from a comparison with the Astrographic Catalogue (AC 2000) and 143 other ground-based astrometric catalogues, all reduced to the Hipparcos

celestial coordinate system In astronomy, coordinate systems are used for specifying positions of celestial objects (satellites, planets, stars, galaxies, etc.) relative to a given reference frame, based on physical reference points available to a situated observer (e. ...

. There were only about 100,000 stars for which proper motion could not be derived. For stars brighter than Vt=9, the astrometric error is 7 milliarcseconds. The overall error for all stars is 60 milliarcseconds. The observational period was from 1989.85 to 1993.21 and the mean satellite observation epoch is 1991.5. Photometric accuracy for stars brighter than Vt=9 is 0.013 magnitude; for all stars it is 0.10 magnitude.

Accessing the catalogue

To enable rapid access of specific stars in the catalogue, WCSTools software numbers each star using its Guide Star region number (0001 to 9537) and a five-digit star number within each region, separated by a decimal point. sty2 lists Tycho-2 stars by number or sky region. imty2 lists the Tycho-2 stars within an IRAF or FITS image using the world coordinate system defined in its header. A

Perl Perl is a high-level, general-purpose, interpreted, dynamic programming language. Though Perl is not officially an acronym, there are various backronyms in use, including "Practical Extraction and Reporting Language". Perl was developed ...

program for extracting data from the catalogue is available from http://archive.eso.org/ASTROM/. Tycho-2 File Formats WCSTools software uses the files catalog.dat and index.dat.

External resources

The following FTP sites make the Tycho-2 catalogue available as 20 gzipped files. After retrieving them and the index.dat file, combine the gzipped files into catalog.dat. The resulting files will differ from the files on the CD-ROM in that they will end lines with only a linefeed, while the CD-ROM version for which WCSTools software was written terminates lines with a carriage return and a linefeed. Add the carriage returns to the files using the unix2dos -ascii command. Put the catalog.dat and index.dat files in a subdirectory data/ to the directory pointed to by the TY2_PATH environment parameter or the ty2cd variable in libwcs/ty2read.c.
Centre de données astronomiques de Strasbourg

References

{{Reflist

External links

The Tycho-2 Catalogue

The Tycho-2 Catalogue as a CSV dataset
Astronomical catalogues of stars Hipparcos

Catalogue

Accessing the catalogue

External resources

See also

References

External links