FK4

FK4 coordinate system is from the Fundamental Catalog of Stars 4.

  3  FK4         Fundamental Catalog (4). The FK4 reference
                      frame is derived from the B1950 frame by
                      applying the equinox offset determined by
                      Fricke.
 
                      [ 0.525" ]

References

• https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/frames.html
• https://en.wikipedia.org/wiki/Catalogues_of_Fundamental_Stars