The Tycho-2 Catalogue is an

astronomical catalogue An astronomical catalog or catalogue is a list or tabulation of astronomical objects, typically grouped together because they share a common type, morphology, origin, means of detection, or method of discovery. The oldest and largest are star c ...

of more than 2.5 million of the brightest stars.

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. His ...

reference catalogue contain positions,

proper motion Proper motion is the astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more dista ...

s, and two-color photometric data for 2,539,913 of the brightest stars in the Milky Way. Components of

double stars In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes. This occurs because the pair either forms a binary star (i.e. a bi ...

with separations down to 0.8 arcseconds 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 , owners = , headquarters = Paris, Île-de-France, France , coordinates = , spaceport = Guiana Space Centre , seal = File:ESA emblem seal.png , seal_size = 130px , image = Views in the Main Control Room (1205 ...

's Hipparcos 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 United States Naval Observatory (USNO) is a scientific and military facility that produces geopositioning, navigation and timekeeping data for the United States Navy and the United States Department of Defense. Established in 1830 as the Depo ...

(USNO) first compiled the ACT Reference Catalog, (Astrographic Catalogue / Tycho) containing nearly one million stars, by combining the

Astrographic Catalogue The Carte du Ciel (literally, 'Map of the Sky') and the Astrographic Catalogue (or Astrographic Chart) were two distinct but connected components of a massive international astronomical project, initiated in the late 19th century, to catalogue an ...

(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 by ...

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 Astronomical coordinate systems are organized arrangements for specifying positions of satellites, planets, stars, galaxies, and other celestial objects relative to physical reference points available to a situated observer (e.g. the true hor ...

. 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 family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offic ...

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 (sometimes named or ) is a tool to convert line breaks in a text file from Unix format (Line feed) to DOS format ( carriage return + Line feed) and vice versa. When invoked as the program will convert a Unix text file to DOS format, when inv ...

-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
Astronomical catalogues of stars Hipparcos

Catalogue

Accessing the catalogue

External resources

See also

References

External links