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Paper III - Spectral Coordinates

These conventions are used when reading a FrameSet from a FitsChan which includes appropriate CTYPEi values, and when writing a FrameSet in which the WCS Frame is a SpecFrame.

Table [*] describes the use made by AST of each keyword whose meaning is defined or extended by FITS-WCS paper III.


Table: 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 TabOK attribute to be set in the FitsChan). Determined by the System attribute of the SpecFrame and the nature of the pixel to SpecFrame Mapping.
CUNITia Used to set the Units attribute of the SpecFrame (note, SpecFrames always have an ``active'' Units attribute (see astSetActiveUnit). Always written.
PVi_ma Used to create the pixel to WCS Mapping (values are stored as attributes of a GrismMap). Set from the attributes of the GrismMap, if present, and if set explicitly.
SPECSYSa Used to set the StdOfRest 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 ObsLon and ObsLat 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 Epoch attributes of the SpecFrame. Set from the Epoch attribute of the SpecFrame, if it has been set explicitly.
SSYSSRCa Used to set the SourceVRF attribute of the SpecFrame (all systems are supported except CMBDIPOL). Set from the SourceVRF attribute of the SpecFrame.
ZSOURCEa Used to set the SourceVel 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 RestFreq 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

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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