## Paper I - General Linear Coordinates

When reading a FrameSetFrameSet from a FitsChanFitsChan, these conventions are used if the CTYPEi keyword values within the FitsChan do not conform to the conventions described in later papers, in which case the axes are assumed to be linear. When writing a FrameSet to a FitsChan, these conventions are used for axes which are described by a simple FrameFrame (i.e. not a SkyFrameSkyFrame, SpecFrameSpecFrame, etc.).

TableTable 1 describes the use made by AST of each keyword defined by FITS-WCS paper I.

Table 1: Use of FITS-WCS Paper I keywords

Keyword | Read | Write

WCSAXESa&Ignored.&Set to the number of axes in the WCS Frame - only written if different to NAXIS.

CRVALia&Used to create the pixel to WCS MappingMapping.&Always written (see “Choice of Reference Point” below).

CRPIXja&Used to create the pixel to WCS Mapping.&Always written (see “Choice of Reference Point” below).

CDELTia&Used to create the pixel to WCS Mapping.&Only written if the CDMatrixCDMatrix attribute of the FitsChan is set to zero.

CROTAi&Used to create the pixel to WCS Mapping.&Only written in FITS-AIPS and FITS-AIPS++ encodings.

CTYPEia&Used to choose the class and attributes of the WCS Frame, and to create the pixel to WCS Mapping (note, “STOKES” and “COMPLEX” axes are treated as unknown linear axes).&Always written (see “Use and Choice of CTYPE keywords” below).

CUNITia&Used to set the Units attributes of the WCS Frame.&Only written if the Units attribute of the WCS Frame has been set explicitly. If so, the Units value for each axis is used as the CUNIT value.

PCi_j a&Used to create the pixel to WCS Mapping.&Only written if the CDMatrix attribute of the FitsChan is set to zero.

CDi_j a&Used to create the pixel to WCS Mapping.&Only written if the CDMatrix attribute of the FitsChan is set to a non-zero value.

PVi_ma&Ignored for linear axes.&Not written if the axes are linear.

PSi_ma&Ignored.&Not used.

WCSNAMEa&Used to set the DomainDomain attribute of the WCS Frame.&Only written if the Domain attribute of the WCS Frame has been set explicitly. If so, the Domain value is used as the WCSNAME value.

CRDERia&Ignored.&Not used.

CSYERia&Ignored.&Not used.

Subsections

- Requirements for a Successful Write Operation
- Use and Choice of CTYPEi keywords
- Choice of Reference Point
- Choice of Axis Ordering
- Alternate Axis Descriptions