The values used are represented by macros with names of the form
" is the (upper case) 3-character code used by the FITS-WCS
" 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).
For a list of available projections, see the FITS-WCS paper.