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Standard methods for specifying world coordinate systems (WCS) in FITS images (Hanisch et al. 2001) have been developed by Greisen & Calabretta (2002) and applied by Calabretta & Greisen (2002) to the problem of celestial coordinates (Papers I & II). The extension to spectral coordinate systems (Greisen et al. 2003, Paper III) is also at an advanced stage.
However, these methods implicitly assume ideal astronomical instrumentation and are not easily adapted to describe the complex distortions found in some imaging devices. Examples abound, from classical ``plate solutions'', to spectrometer wavelength calibration. It may also happen that the distortion is ``inherent'' to the object of study, for example the oblateness of the Sun and Earth. Irregularity of form is carried to an extreme by various minor bodies of the Solar System.
Work in progress, summarised here, extends the current FITS WCS formalism by providing methods to describe the distortions inherent in the image coordinate systems of real astronomical data. It is envisaged that a range of distortion functions will be provided, including N-dimensional polynomial, cubic spline, B-spline, and table lookup methods; that these will be applicable over multiple image dimensions; and that they may be applied either before or after the standard linear transformation stage of the coordinate calculation.
An early draft of Paper IV is available for comment. We invite input from the general FITS user community regarding the adequacy of the proposed methods for existing or future applications.
The new steps to be introduced into the algorithm chain are enclosed in dashed boxes in Figure 1. Key features are:
The Australia Telescope is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO.
The National Optical Astronomy Observatory is a facility of the (U.S.) National Science Foundation operated under cooperative agreement by Associated Universities for Research in Astronomy, Inc.
The National Radio Astronomy Observatory is a facility of the (U.S.) National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
UCO/Lick Observatory is operated by the University of California.
Calabretta, M. R. & Greisen, E. W. 2002, A&A, 395, 1075 (Paper II)
Greisen, E. W. & Calabretta, M. R. 2002, A&A, 395, 1059 (Paper I)
Greisen, E. W., Valdes, F. G., Calabretta, M. R. & Allen, S. L. 2003, A&A, in preparation, Representations of spectral coordinates in FITS (Paper III)
Hanisch, R. J., Farris, A., Greisen, E. W., Pence, W. D., Schlesinger, B. M., Teuben, P. J., Thompson, R. W., & Warnock III, A. 2001, A&A, 376, 359