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FitsWcsCoverageFITS-WCS Coverage

This appendix gives details of the FitsChanFitsChan 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 EncodingEncoding attribute of the FitsChan has the value “FITS-WCS” (whether set explicitly or defaulted). It should always be possible for a FrameSetFrameSet to be read (using the astReadastRead 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 astWriteastWrite function. For instance, if the current FrameFrame of a FrameSet is re-mapped using, say, an arbitrary MathMapMathMap 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.

Subsections

- Paper I - General Linear Coordinates
  - Requirements for a Successful Write Operation
  - Use and Choice of CTYPEi keywords
  - Choice of Reference Point
  - Choice of Axis Ordering
  - Alternate Axis Descriptions
- Paper II - Celestial Coordinates
  - Requirements for a Successful Write Operation
  - Choice of LONPOLE/LATPOLE
  - User Defined Fiducial Points
  - Common Non-Standard Features
- Paper III - Spectral Coordinates
  - Requirements for a Successful Write Operation
  - Common Non-Standard Features
- Paper IV - Coordinate Distortions
  - The “-SIP” distortion code