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When writing a FrameSet in which the WCS
Frame is a SkyFrame to a
FitsChan, success depends on the following conditions
being met:
- The Mapping from pixel coordinates (the base Frame
in the FrameSet) to the WCS SkyFrame includes a WcsMap.
- The Mapping prior to the WcsMap (i.e. from pixel to IWC) is linear.
- The Mapping after the WcsMap (i.e. from native spherical to
celestial coordinates) is a spherical rotation for the
celestial axes, and linear for any other axes.
- The TabOK attribute is set to a non-zero positive value in the FitsChan,
and the longitude and latitude axes are separable. In this case the Mapping will
be described by a pair of 1-dimensional look-up tables, using the ``-TAB''
algorithm described in FITS-WCS paper III.
If none of the above conditions hold, the write operation will be
unsuccessful.
Next: Choice of LONPOLE/LATPOLE
Up: Paper II - Celestial Coordinates
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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