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CarLin - Ignore spherical rotations on CAR projections?

Description:
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. Furthermore, if CarLin is non-zero, it is assumed that CDELT and CD keywords are in units of degrees rather than radians (as required by the FITS-WCS papers).

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.

Type:
Integer (boolean).
Class Applicability:
FitsChan
All FitsChans have this attribute.


next up previous 264
Next: Card - Index of current FITS card in a FitsChan
Up: AST Attribute Descriptions
Previous: CDMatrix - Use CDi_j keywords to represent pixel scaling, rotation, etc?

AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 210
R.F. Warren-Smith & D.S. Berry
24th May 2011
E-mail:ussc@star.rl.ac.uk

Copyright (C) 2009 Science \& Technology Facilities Council