2. Main catalogue and two supplements
The Tycho-2 Catalogue contains positions and magnitudes for stars. We normally give two positions for each star, viz. the observedposition and the meanposition. For all stars we give the observed position , based on satellite data only, at the Tycho observation epoch which is close to 1991.5. For 96 % of the stars, at least one ground-based observation was available and a proper motion was derived. The weighted least-squares solution giving the proper motion, also gave the mean position at the mean epoch. But in the catalogue we give the mean position, rigorously propagated to the epoch J2000.0 by means of the proper motion.
Stars brighter than about or could not be treated properly and are excluded from the catalogue. For the convenience of the user, stars from the Hipparcos and Tycho-1 Catalogues which - for various reasons - are not included in the Tycho-2 Catalogue, have been listed in supplementary catalogues giving astrometry and photometry from either the Hipparcos or the Tycho-1 Catalogue. The first supplement contains good quality stars while the second supplement contains 1146 Tycho-1 stars which are either false or heavily disturbed by brighter stars. Any star from Tycho-1 marked with an astrometric quality flag of `9' and which was not detected in Tycho-2 processing was excluded from both supplements.
The catalogue provides cross references to the Tycho-1 Catalogue and to stars in the Hipparcos Catalogue and its annex of double and multiple stars with component solutions. In general a distance limit of 0.8 arcsec was used for the identification.
2.1. TYC numbers
For Tycho-2 we use the same system of star numbers as in Tycho-1, and the numbers are consistent. The system is basically the same as used in GSC, the Guide Star Catalog (Jenkner et al. 1990), but with a component number appended.
Each Tycho-2 entry, e.g. TYC 5311-1245-1, has a set of three identification numbers: TYC1, TYC2 and TYC3. TYC1 is the GSC region number, TYC2 is the running number within the region, and TYC3 is a Tycho specific component number. Double star components will either have two different running numbers or the same running number and different component numbers.
In about 4 500 cases, new consistent running numbers within GSC regions were constructed for Tycho-2.
2.2. The proximity indicator
When selecting stars from a catalogue like Tycho-2, it is often useful to know if there are other stars present in the neighbourhood. For this purpose we give a proximity indicator for each star, which is simply the distance to the nearest neighbour in the main catalogue or in the main supplement. Stars in the second supplement, i.e. false Tycho-1 stars, were ignored.
© European Southern Observatory (ESO) 2000
Online publication: March 9, 2000