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PVj_m - FITS-WCS projection parameters   

Description:
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, j and m, separated by an underscore. The axis index is specified by j, 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 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.

Type:
Floating point.

Class Applicability:
WcsMap
All WcsMaps have this attribute.

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.



next up previous
Next: PcdCen(axis) - Centre coordinates of pincushion/barrel distortion
Up: AST Attribute Descriptions
Previous: NumLabGap(axis) - Spacing of numerical labels for a Plot

AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 211
R.F. Warren-Smith & D.S. Berry
30th April 2003
E-mail:ussc@star.rl.ac.uk

Copyright (C) 2003 Central Laboratory of the Research Councils