The values used are represented by macros with names of the form "AST__XXX", where "XXX" is the (upper case) 3-character code used by the FITS-WCS "CTYPEi" keyword to identify the projection. For example, possible values are AST__TAN (for the tangent plane or gnomonic projection) and AST__AIT (for the Hammer-Aitoff projection). AST__TPN is an exception in that it is not part of the FITS-WCS standard (it represents a TAN projection with polynomial correction terms as defined in an early draft of the FITS-WCS paper).
AST A Library for Handling World Coordinate Systems in Astronomy