This paper proposes that an additional 4 character code be appended to the end of the CTYPEi keyword to specify the nature of any distortion away from the basic algorithm described by the first 8 characters of the CTYPEi value. Currently AST ignores all such codes when reading a FrameSet from a FitsChan (except for the ``-SIP'' code defined by the Spitzer Space Telescope project - see below). This means that a FrameSet can still be read from such headers, but the Mapping which gives the WCS position associated with a given pixel position will reflect only the basic algorithm and will not include the effects of the distortion.
If such a FrameSet is then written out to a FitsChan, the resulting CTYPEi keywords will include no distortion code.
AST A Library for Handling World Coordinate Systems in Astronomy