Reference Frames
The frames that anchor coordinates in space — the FK4 and FK5 fundamental catalogues and the quasar-based ICRF3 that realises the modern ICRS.
- The Fifth Fundamental Catalogue (FK5)Reference frame
A fundamental reference frame published in 1988, giving precise positions and proper motions for 1,535 bright stars on the equinox and epoch J2000. FK5 was the optical standard of its era, but it rested on a limited number of stars; it was superseded when the ICRS, realised by Hipparcos and later Gaia, replaced star-based frames with an extragalactic one.
- The Fourth Fundamental Catalogue (FK4)Reference frame
The predecessor of FK5, a fundamental star frame referred to the equinox and epoch B1950. Positions given in the FK4 (B1950) system must be precessed and rotated to be compared with modern J2000/ICRS coordinates — a common source of small errors when working with older catalogues and charts.
- The International Celestial Reference Frame (ICRF3)Reference frame
The practical realisation of the International Celestial Reference System — a catalogue of precise positions for several thousand extragalactic radio sources, mostly quasars, measured by very-long-baseline interferometry. Its third realisation, ICRF3, was adopted in 2018; because quasars are effectively fixed, it provides the quasi-inertial grid to which optical frames such as Gaia's are aligned.