Catalogues of Fundamental Stars

The Catalogue of Fundamental Stars is a series of six astrometric catalogues of high precision positional data for a small selection of stars to define a celestial reference frame, which is a standard coordinate system for measuring positions of stars.

The Fourth Fundamental Catalogue (FK4) was published in 1963, and contained 1,535 stars in various equinoxes from 1950.0 to 1975.0 .

The Fourth Fundamental Catalogue's Supplement (FK4S) was an amendment to FK4 that contains a further 1,987 stars.

The Fifth Fundamental Catalogue (FK5) was a 1988 update of FK4 with new positions for the 1,535 stars. It was superseded by the quasar-based International Celestial Reference Frame (ICRF).

The Fifth Fundamental Catalogue Extension (FK5), published in 1991, added 3,117 new stars.

The Sixth Fundamental Catalogue (FK6) is a 2000 update of FK5 correlated with the ICRF through the Hipparcos satellite. It comes in two parts: FK6(I) and FK6(III). FK6(I) contains 878 stars, and FK6(III) contains 3,272 stars. Both are updated and amended versions of FK5, using Hipparcos catalogue data.

Astronomical Calculation Institute

...
Wikipedia