COCO

 COCO is an interactive program that accepts a celestial position in one reference system and transforms it into another reference system.

Eight reference systems are supported:

  • mean [α,δ], FK4 (barycentric)
  • like FK4 but without E-terms (barycentric)
  • mean [α,δ], FK6 (barycentric)
  • ICRS [α,δ]
  • CIRS [α,δ] (geocentric)
  • geocentric apparent (α,δ)
  • ecliptic [λ,β]
  • galactic [lII,bII]

    Here are the COCO commands required to transform the ICRS coordinates of the BL Lac object QSO B0446+113 into FK4 coordinates: 

    i c
o 4
04 49 07.67110194 +11 21 28.5963618

    What is seen on the user's terminal: 

    <- i c

   Conversion is from:
      ICRS, epoch J2000.00, (barycentric)

<- o 4

   Conversion is to:
      FK4, equinox B1950.0, epoch B1950.00

<- 04 49 07.67110194 +11 21 28.5963618

 = 04 49 07.671 +11 21 28.60  J2000.00  ICRS (barycentric)
-> 04 46 21.220 +11 16 17.81  B1950.00  B1950.00  FK4

    The application makes heavy use of the IAU SOFA software. Depending on which transformation is being carried out, accuracy is milliarcsecond or better.

    COCO is a modern implementation of a tool developed by the UK Starlink project in the 1990s.

    The user guide is coco.pdf and the files are in coco.zip

    Patrick Wallace, 2-Sep-2025