Ignore spherical rotations on CAR projections?
"
CAR"
(plate carree, or "
Cartesian"
) projections should be treated
when reading a FrameSet from a foreign encoded FITS header. If zero (the default), it is assumed that
the CAR projection conforms to the conventions described in the FITS world coordinate system
(FITS-WCS) paper II "
Representation of Celestial Coordinates in FITS"
by M. Calabretta & E.W.
Greisen. If CarLin is non-zero, then these conventions are ignored, and it is assumed that the mapping
from pixel coordinates to celestial coordinates is a simple linear transformation (hence the attribute
name "
CarLin"
). This is appropriate for some older FITS data which claims to have a "
CAR"
projection, but which in fact do not conform to the conventions of the FITS-WCS
paper.
The FITS-WCS paper specifies that headers which include a CAR projection represent a linear
mapping from pixel coordinates to "
native spherical coordinates"
, NOT celestial coordinates. An
extra mapping is then required from native spherical to celestial. This mapping is a 3D rotation and so
the overall Mapping from pixel to celestial coordinates is NOT linear. See the FITS-WCS papers for
further details.