## Paper III - Spectral Coordinates

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

TableTable 3 describes the use made by AST of each keyword whose meaning is defined or extended by FITS-WCS paper III.

Table 3: Use of FITS-WCS Paper III keywords

Keyword | Read | Write

CTYPEia&All coordinate systems and projection types listed in paper III are supported algorithm (the “-LOG” algorithm may also be applied to non-spectral linear axes; the “-TAB” algorithm requires the TabOKTabOK attribute to be set in the FitsChan).&Determined by the SystemSystem attribute of the SpecFrame and the nature of the pixel to SpecFrame MappingMapping.

CUNITia&Used to set the Units attribute of the SpecFrame (note, SpecFrames always have an “active” Units attribute (see astSetActiveUnitastSetActiveUnit).&Always written.

PVi_ma&Used to create the pixel to WCS Mapping (values are stored as attributes of a GrismMapGrismMap).&Set from the attributes of the GrismMap, if present, and if set explicitly.

SPECSYSa&Used to set the StdOfRestStdOfRest attribute of the SpecFrame (all systems are supported except CMBDIPOL).&Set from the StdOfRest attribute of the SpecFrame, but only if it has been set explicitly.

SSYSOBSa&Ignored.&Never written.

OBSGEO-X/Y/Z&Used to set the ObsLonObsLon and ObsLatObsLat attributes of the Frame (the observers height above sea level is ignored).&Set from the ObsLon and ObsLat attributes of the Frame, if they have been set explicitly (it is assumed that the observer is at sea level).

MJD-AVG&Used to set the EpochEpoch attributes of the SpecFrame.&Set from the Epoch attribute of the SpecFrame, if it has been set explicitly.

SSYSSRCa&Used to set the SourceVRFSourceVRF attribute of the SpecFrame (all systems are supported except CMBDIPOL).&Set from the SourceVRF attribute of the SpecFrame.

ZSOURCEa&Used to set the SourceVelSourceVel attribute of the SpecFrame (the SourceVRF attribute is first set to the system indicated by the SSYSSRC keyword, and the ZSOURCE value is then converted to an apparent radial velocity and stored as the SourceVel attribute).&Set from the SourceVel attribute of the SpecFrame, if it has been set explicitly (the SourceVel value is first converted from apparent radial velocity to redshift).

VELOSYSa&Ignored.&Set from the attributes of the SpecFrame that define the standard of rest and the observers position.

RESTFRQa&Used to set the RestFreqRestFreq attribute of the SpecFrame.&Set from the RestFreq attribute of the SpecFrame, but only if the System attribute is not set to “WAVE”, “VOPT”, “ZOPT” or “AWAV”, and only if RestFreq has been set explicitly.

RESTWAVa&Used to set the RestFreq attribute of the SpecFrame (after conversion from wavelength to frequency).&Set from the RestFreq attribute of the SpecFrame (after conversion), but only if the System attribute is set to “WAVE”, “VOPT”, “ZOPT” or “AWAV”, and only if RestFreq has been set explicitly.

CNAMEia&Used to set the Label attributes of the WCS Frame keywords.&Set from the Label attributes of the WCS Frame, if they have been set explicitly.

Subsections

- Requirements for a Successful Write Operation
- Common Non-Standard Features