This attribute is a boolean value which specifies how FITS " 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.

FitsChan