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