## Paper IV - Coordinate Distortions

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 FrameSetFrameSet from a FitsChanFitsChan (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 MappingMapping 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.

Subsections

- The “-SIP” distortion code