Paper II - Celestial Coordinates

These conventions are used when reading a FrameSetFrameSet from a FitsChanFitsChan containing appropriate CTYPEi values, and when writing a FrameSet in which the WCS FrameFrame is a SkyFrameSkyFrame.

TableTable 2 describes the use made by AST of each keyword whose meaning is defined or extended by FITS-WCS paper II.


Table 2: Use of FITS-WCS Paper II keywords
Keyword Read Write
CTYPEia All coordinate systems and projection types listed in paper II are supported (note, ``CUBEFACE'' axes are treated as unknown linear axes). In addition, "-HPX" (HEALPix) and "-XPH" (polar HEALPix) are supported. Determined by the SystemSystem attribute of the SkyFrame and the WcsTypeWcsType attribute of the WcsMapWcsMap within the FrameSet.
CUNITia Ignored (assumed to be 'degrees'). Not written.
PVi_ma Used to create the pixel to WCS MappingMapping (values are stored as attributes of a WcsMap within this Mapping). Values are obtained from the WcsMap in the pixel to WCS Mapping.
LONPOLEa Used to create the pixel to WCS Mapping. Also stored as a PVi_mPVi_m attribute for the longitude axis of the WcsMap. Only written if not equal to the default value defined in paper II (see ``Choice of LONPOLE/LATPOLE'' below).
LATPOLEa Used to create the pixel to WCS Mapping. Also stored as a PV attribute for the longitude axis of the WcsMap. Only written if not equal to the default value defined in paper II (see ``Choice of LONPOLE/LATPOLE'' below).
RADESYSa Used to set the attributes of the SkyFrame. All values supported except that ecliptic coordinates are currently always assumed to be FK5. Always written. Determined by the System attribute of the SkyFrame.
EQUINOXa Used to set the EquinoxEquinox attribute of the SkyFrame. Written if relevant. Determined by the Equinox attribute of the SkyFrame.
EPOCH Used to set the Equinox attribute of the SkyFrame. Only written if using FITS-AIPS and FITS-AIPS++ encodings. Determined by the Equinox attribute of the SkyFrame.
MJD-OBS Used to set the EpochEpoch attribute of the SkyFrame. DATE-OBS is used if MJD-OBS is not present. A default value based on RADESYS and EQUINOX is used if used if DATE-OBS is not present either. Determined by the Epoch attribute of the SkyFrame. Only written if this attribute has been set to an explicit value (in which case DATE-OBS is also written).





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