... Calabretta, M. R., Valdes, F., Greisen, E. W., & Allen, S. L. 2003, in ASP Conf. ... Of particular interest to the fields of astrometry and spectroscopy, the fourth in a series of papers defining conventions for encoding world coordinate information in FITS headers will consider the problem of representing small systematic errors, or distortions. ... 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) ...