This appendix gives details of the FitsChan class implementation of the conventions described in the FITS-WCS papers available at http://fits.gsfc.nasa.gov/fits_wcs.html. These conventions are used only if the Encoding attribute of the FitsChan has the value ``FITS-WCS'' (whether set explicitly or defaulted). It should always be possible for a FrameSet to be read (using the astRead function) from a FitsChan containing a header which conforms to these conventions. However, only those FrameSets which are compatible with the FITS-WCS model can be written to a FitsChan using the astWrite function. For instance, if the current Frame of a FrameSet is re-mapped using, say, an arbitrary MathMap then the FrameSet will no longer be compatible with the FITS-WCS model, and so will not be written out successfully to a FitsChan.
The following sub-sections describe the details of the implementation of
each of the first four FITS-WCS papers. Here, the term ``pixel axes'' is
used to refer to the FITS pixel coordinates (i.e. the centre of the
first image pixel has a value 1.0 on each pixel axis); the term ``IWC
axes'' is used to refer to the axes of the Intermediate World Coordinate
system; and the term ``WCS axes'' is used to refer to the axes of the final
physical coordinate system described by the CTYPEi keywords.
AST A Library for Handling World Coordinate Systems in Astronomy