Implement a FITS-WCS sky projection
"
Representations of Celestial Coordinates in FITS"
by M. Calabretta and E.W. Griesen. This paper
defines a set of functions, or sky projections, which transform longitude-latitude pairs representing
spherical celestial coordinates into corresponding pairs of Cartesian coordinates (and vice
versa).
A WcsMap is a specialised form of Mapping which implements these sky projections and applies
them to a specified pair of coordinates. All the projections in the FITS-WCS paper are supported, plus
the now deprecated "
TAN with polynomial correction terms"
projection which is refered to here by
the code "
TPN"
. Using the FITS-WCS terminology, the transformation is between "
native spherical"
and "
projection plane"
coordinates (also called "
intermediate world coordinates"
. These coordinates
may, optionally, be embedded in a space with more than two dimensions, the remaining coordinates
being copied unchanged. Note, however, that for consistency with other AST facilities, a
WcsMap handles coordinates that represent angles in radians (rather than the degrees used by
FITS-WCS).
The type of FITS-WCS projection to be used and the coordinates (axes) to which it applies are specified when a WcsMap is first created. The projection type may subsequently be determined using the WcsType attribute and the coordinates on which it acts may be determined using the WcsAxis(lonlat) attribute.
Each WcsMap also allows up to 100 "
projection parameters"
to be associated with each axis. These
specify the precise form of the projection, and are accessed using PVi_m attribute, where "
i"
is the
integer axis index (starting at 1), and m is an integer "
parameter index"
in the range 0 to 99. The
number of projection parameters required by each projection, and their meanings, are dependent
upon the projection type (most projections either do not use any projection parameters, or use
parameters 1 and 2 associated with the latitude axis). Before creating a WcsMap you should consult
the FITS-WCS paper for details of which projection parameters are required, and which have
defaults. When creating the WcsMap, you must explicitly set values for all those required
projection parameters which do not have defaults defined in this paper.
NatLat: Native latitude of the reference point of a FITS-WCS projection
NatLon: Native longitude of the reference point of a FITS-WCS projection
PVi_m: FITS-WCS projection parameters
PVMax: Maximum number of FITS-WCS projection parameters
WcsAxis(lonlat): FITS-WCS projection axes
WcsType: FITS-WCS projection type