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Use and Choice of CTYPEi keywords

When reading a FrameSet from a FitsChan the CTYPEi values in the FitsChan are used to set the Symbol attributes of the corresponding WCS Frame. The Label attributes of the WCS Frame are set from the CNAMEi keywords, if present in the header. Otherwise they are set from the CTYPEi comments strings in the header, so long as each axis has a unique non-blank comment. Otherwise, the Label attributes are set to the CTYPEi values. The above procedure is over-ridden if the axis types conform to the conventions described in paper II or III, as described below.

When writing a FrameSet to a FitsChan, each CTYPEi value is set to the value of the Symbol attribute of the corresponding axis in the Frame being written. If a value has been set explicitly for the axis Label attribute, it is used as the axis comment (except that any existing comments in the FitsChan take precedence if the keyword value has not changed). The above procedure is over-ridden if the Frame is a SkyFrame or a SpecFrame, in which case the CTYPEi value is derived from the System attribute of the Frame and the nature of the pixel to WCS Mapping according to the conventions of papers II and III, as described below.



next up previous 264
Next: Choice of Reference Point
Up: Paper I - General Linear Coordinates
Previous: Requirements for a Successful Write Operation

AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 210
R.F. Warren-Smith & D.S. Berry
24th May 2011
E-mail:ussc@star.rl.ac.uk

Copyright (C) 2009 Science \& Technology Facilities Council