## 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).

Subsections

- Requirements for a Successful Write Operation
- Choice of LONPOLE/LATPOLE
- User Defined Fiducial Points
- Common Non-Standard Features