WcsMap Attributes

NatLat

Native latitude of the reference point of a FITS-WCS projection. (double, read-only)

This attribute gives the latitude of the reference point of the FITS-WCS projection implemented by a WcsMap. The value is in radians in the "native spherical" coordinate system. This value is fixed for most projections, for instance it is PI/2 (90 degrees) for all zenithal projections. For some projections (e.g. the conics) the value is not fixed, but is specified by parameter one on the latitude axis.

FITS-WCS paper II introduces the concept of a "fiducial point" which is logical distinct from the projection reference point. It is easy to confuse the use of these two points. The fiducial point is the point which has celestial coordinates given by the CRVAL FITS keywords. The native spherical coordinates for this point default to the values of the NatLat and NatLon {NatLon" attributes, but these defaults mey be over-ridden by values stored in the PVi_j keywords. Put another way, the CRVAL keywords will by default give the celestial coordinates of the projection reference point, but may refer to some other point if alternative native longitude and latitude values are provided through the PVi_j keywords.

NatLon

Native longitude of the reference point of a FITS-WCS projection. (double, read-only)

This attribute gives the longitude of the reference point of the FITS-WCS projection implemented by a WcsMap. The value is in radians in the "native spherical" coordinate system, and will usually be zero. See NatLat for further information.

PVi_m

FITS-WCS projection parameters. (double)

This attribute specifies the projection parameter values to be used by a WcsMap when implementing a FITS-WCS sky projection. Each PV attribute name should include two integers, i and m, separated by an underscore. The axis index is specified by i, and should be in the range 1 to 99. The parameter number is specified by m, and should be in the range 0 to 99. For example, "PV2_1=45.0" would specify a value for projection parameter 1 of axis 2 in a WcsMap.

These projection parameters correspond exactly to the values stored using the FITS-WCS keywords "PV1_1", "PV1_2", etc. This means that projection parameters which correspond to angles must be given in degrees (despite the fact that the angular coordinates and other attributes used by a WcsMap are in radians).

The set of projection parameters used by a WcsMap depends on the type of projection, which is determined by its WcsType parameter. Most projections either do not require projection parameters, or use parameters 1 and 2 associated with the latitude axis. You should consult the FITS-WCS paper for details.

Some projection parameters have default values (as defined in the FITS-WCS paper) which apply if no explicit value is given. You may omit setting a value for these "optional" parameters and the default will apply. Some projection parameters, however, have no default and a value must be explicitly supplied. This is most conveniently done using the "options" argument of astWcsMap (q.v.) when a WcsMap done using the OPTIONS argument of AST_WCSMAP (q.v.) when a WcsMap is first created. An error will result when a WcsMap is used to transform coordinates if any of its required projection parameters has not been set and lacks a default value.

A "get" operation for a parameter which has not been assigned a value will return the default value defined in the FITS-WCS paper, or AST__BAD if the paper indicates that the parameter has no default. A default value of zero is returned for parameters which are not accessed by the projection.

Note, the FITS-WCS paper reserves parameters 1 and 2 on the longitude axis to hold the native longitude and latitude of the fiducial point of the projection, in degrees. The default values for these parameters are determined by the projection type. The AST-specific TPN projection does not use this convention - all projection parameters for both axes are used to represent polynomical correction terms, and the native longitude and latitude at the fiducial point may not be changed from the default values of zero and 90 degrees.

Notes:

If the projection parameter values given for a WcsMap do not satisfy all the required constraints (as defined in the FITS-WCS paper), then an error will result when the WcsMap is used to transform coordinates.

PVMax(i)

Maximum number of FITS-WCS projection parameters. (int, read-only)

This attribute specifies the largest legal index for a PV projection parameter attached to a specified axis of the WcsMap (i.e. the largest legal value for m when accessing the PVi_m attribute). The axis index is specified by i, and should be in the range 1 to 99. The value for each axis is determined by the projection type specified when the WcsMap is first created cannot subsequently be changed.

WcsAxis(lonlat)

FITS-WCS projection axes. (int, read-only)

This attribute gives the indices of the longitude and latitude coordinates of the FITS-WCS projection within the coordinate space used by a WcsMap. These indices are defined when the WcsMap is first created and cannot subsequently be altered.

If lonlat is 1, the index of the longitude axis is returned. If lonlat is 2, the index of the latitude axis is returned.

WcsType

FITS-WCS projection type. (int, read only)

This attribute specifies which type of FITS-WCS projection will be performed by a WcsMap. The value is specified when a WcsMap is first created and cannot subsequently be changed.

The values used are represented by enum class WcsType with names that match the (upper case) 3-character code used by the FITS-WCS CTYPEi keyword to identify the projection. For example WcsType::TAN is the tangent plane or gnomonic projection and WcsType::AIT is the Hammer-Aitoff projection. WcsType::TPN is an exception in that it is not part of the FITS-WCS standard (it represents a TAN projection with polynomial correction terms as defined in an early draft of the FITS-WCS paper).

Notes:

For a description of available projections, see the FITS-WCS paper.