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Description
The FK6 is a suitable combination of the results of the HIPPARCOS (Cat. <I/239>) astrometry satellite with ground-based data, measured over more than two centuries. The FK6 gives other solutions than the classical `single-star mode' (SI mode), which take into account the fact that hidden astrometric binaries among `apparently single-stars' introduce sizable `cosmic errors' into the quasi-instantaneously measured HIPPARCOS proper motions and positions. The FK6 gives in addition to the SI mode the `long-term prediction (LTP) mode' and the `short-term prediction (STP) mode'. These LTP and STP modes are on average the most precise solutions for apparently single stars, depending on the epoch difference with respect to the HIPPARCOS epoch of about 1991. The FK6(I) results from the combination of FK5(I) (Cat. <I/149>) with Hipparcos results. Among its 878 stars, it contains 340 objects classified as `astrometrically excellent stars', most suited for high-precision astrometry. The typical mean error of an FK6(I) proper motion is 0.35 mas/year in the single-star mode (about two times better than Hipparcos), and 0.50 mas/year in the long-term prediction mode (about four times better than Hipparcos). The FK6(III) results from the combination of stars from FK5 extension (Cat. <I/175>) with Hipparcos. Among its 3272 stars, it contains 1928 objects classified as `astrometrically excellent stars'. The typical mean error of an FK6(III) proper motion is 0.59 mas/year in the single-star mode, and 0.93 mas/year in the long-term prediction mode.
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