Published in Astronomy & Astrophysics 395, 1059, 2002.

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E. W. Greisen (1) - M. R. Calabretta (2)

(1) National Radio Astronomy Observatory, P. O. Box O, Socorro, NM, USA 87801-0387
(2) Australia Telescope National Facility, P. O. Box 76, Epping, NSW 1710 Australia

Abstract. The initial descriptions of the FITS format provided a simplified method for describing the physical coordinate values of the image pixels, but deliberately did not specify any of the detailed conventions required to convey the complexities of actual image coordinates. Building on conventions in wide use within astronomy, this paper proposes general extensions to the original methods for describing the world coordinates of FITS data. In subsequent papers, we apply these general conventions to the methods by which spherical coordinates may be projected onto a two-dimensional plane and to frequency/wavelength/velocity coordinates. Methods: data analysis - Techniques: image processing - Astronomical data bases: miscellaneous

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