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When writing a FrameSet to a FitsChan,
the choice of LONPOLE and LATPOLE values is determined as follows:
- If the projection represented by the WcsMap is
azimuthal, then any values set for attributes ``PVi_3''
and ``PVi_4'' (where ``i'' is the index of the longitude axis)
within the WcsMap are used as the LONPOLE and LATPOLE values. Reading a
FrameSet from a FITS-WCS header
results in the original LONPOLE and LATPOLE values being stored within a
WcsMap within the FrameSet. Consequently, if a FrameSet is read from a
FITS-WCS header and it is subsequently written out to a new FITS-WCS
header, the original LONPOLE and LATPOLE values will usually be used in
the new header (the exception being if the WcsMap has been explicitly
modified before being written out again). Any extra rotation of the sky
is absorbed into the CDi_j or PCi_j matrix (this is
possible only if the projection is azimuthal).
- If the projection represented by the WcsMap is azimuthal but no
values have been set for the ``PVi_3'' and ``PVi_4''
attributes within the WcsMap, then the default LONPOLE and LATPOLE values
are used. This results in no LONPOLE or LATPOLE keywords being stored in
the header since default values are never stored. Any extra rotation of
the sky is absorbed into the CDi_j or PCi_j matrix (this
is possible only if the projection is azimuthal).
- If the projection represented by the WcsMap is not azimuthal,
then the values of LONPOLE and LATPOLE are found by transforming the
coordinates of the celestial north pole (i.e longitude zero,
latitude
) into native spherical coordinates using the inverse of
the Mapping which follows the WcsMap.
Next: User Defined Fiducial Points
Up: Paper II - Celestial 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