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Astron. Astrophys. 360, 391-398 (2000)

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2. Catalogues

2.1. Hipparcos and Tycho

The HIP and TYC catalogues are the culmination of the Hipparcos astrometry satellite project. Hipparcos had a mission life-time of [FORMULA]four years and ended its observations in 1993. The HIP Catalogue contains high-precision astrometry ([FORMULA]1 mas in position and parallax and [FORMULA]1 mas yr-1 in proper motion) for [FORMULA] stars. The catalogue is complete to [FORMULA] mag and has a magnitude limit of [FORMULA] mag. In contrast, the accuracy of the TYC Catalogue is only [FORMULA]25 mas(yr-1) for position, parallax, and proper motion. However, it consists of [FORMULA] stars, is complete to [FORMULA] mag, and has a magnitude limit of [FORMULA] mag. The [FORMULA] band is one of the two broad-band photometric filters of the Tycho experiment, and is similar to the Johnson V filter. A comprehensive description of the satellite and the construction of the catalogues can be found in ESA (1997).

2.2. TRC and ACT

The "Tycho Reference Catalogue" (TRC) and the "Astrographic Catalogue plus Tycho Reference Catalogue" (ACT) both combine the positional information in the Astrographic Catalogue (Epoch [FORMULA]1910) with that in the Tycho Catalogue (Epoch 1991.25) to obtain proper motions. The Astrographic Catalogue is the result of the Carte du Ciel project, started at the end of the 19th century, with the goal of constructing an all-sky astrometric catalogue complete to photographic magnitude [FORMULA] mag (see, e.g., Eichhorn 1974, Débarbat et al. 1988, and Urban & Corbin 1998). Twenty different observatories participated in this enormous project, each covering its own strip in declination. The TRC Catalogue was constructed by a European consortium (Kuzmin et al. 1999) and the ACT Catalogue by the U.S. Naval Observatory (Urban et al. 1998). Each consortium constructed its own machine-readable version of the Astrographic Catalogue from the 254 printed volumes of raw data. They then used standard photographic-plate reduction techniques to produce a version of the Astrographic Catalogue on the International Celestial Reference System (ICRS) (i.e., the Hipparcos reference system, see also ESA 1997). The mean epoch of the Astrographic Catalogue is [FORMULA]1910. The positions in the Astrographic Catalogue were combined with the Tycho positions of epoch 1991.25 to obtain proper motions. The AC position for a star situated in an overlap region between two adjacent zones is the average of the positions in both zones. The exact details of this entire procedure differ between the two consortia.

The TRC and ACT list proper motions with an accuracy of [FORMULA]3 mas yr-1 for [FORMULA] and [FORMULA], respectively. Note that proper motions in Right Ascension listed in the ACT Catalogue are not multiplied by [FORMULA] (where [FORMULA] is the declination) while the proper-motion errors in Right Ascension are multiplied by [FORMULA] (Urban 1999).

2.3. Tycho 2

A new version of the Tycho Catalogue, the Tycho 2 Catalogue, was released in February 2000. Through a careful re-analysis of the raw Tycho data it was possible to more than double the number of stars in the Catalogue to 2.5 million stars (Hog et al. 2000a, 2000b). The proper motions on the Tycho 2 Catalogue have been constructed in a similar manner as for the TRC and ACT catalogues. The main difference between the TRC/ACT and Tycho 2 is that besides the Astrographic Catalogue, 143 other transit and astrographic catalogues have been used to obtain the proper motions (see Hog et al. 2000b). The average accuracy of the proper motions is [FORMULA]2.5 mas yr-1.

Although the TRC and ACT have now been superseded, we feel that a comparison of these catalogues with the Hipparcos Catalogue is warranted. Both the TRC and ACT have been used extensively and an assessment of the quality of these catalogues will help interpreting the results of these studies. We do not discuss the Tycho 2 catalogue in this paper.

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000
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