1 Introduction

The COCO program converts star coordinates from one system to another. Both the improved IAU system, post-1976, and the old pre-1976 system are supported. COCO can perform accurate transformations between the following coordinate systems:

• mean $\left[\alpha ,\delta \right]$, old system, with E-terms (loosely FK4, usually B1950)
• mean $\left[\alpha ,\delta \right]$, old system, no E-terms (some radio positions)
• mean $\left[\alpha ,\delta \right]$, new system (loosely FK5, usually J2000)
• geocentric apparent $\left[\alpha ,\delta \right]$, new system
• ecliptic coordinates $\left[\lambda ,\beta \right]$, new system (mean of date)
• galactic coordinates $\left[l^{II},b^{II}\right]$, IAU 1958 system

COCO’s user-interface is spartan but efficient. The program offers control over report resolution, and there has a simple online help facility. All input is free-format, and defaults are provided where this is meaningful.

The input/output arrangements of COCO are flexible, to allow a variety of operating styles – interactive, input from a file, report to a file, batch, etc. Also, in addition to the report which is produced, the results of the conversions are also available in a raw form, to a fixed resolution and free from extraneous formatting; this file is intended to be read easily by other programs.

In order to comply with the IAU 1976 recommendations, all position data published from 1984 on should be given in the new system, using equinox J2000.0. However, positions are still frequently given in the old system, using equinox B1950.0. Discriminating astronomers and astrophysicists are best advised to give both B1950 and J2000 positions for sources mentioned in their publications; the conversion can be done using COCO.