A GrismMap is a specialised form of Mapping which transforms 1-dimensional coordinates
using the spectral dispersion equation described in FITS-WCS paper III " Representation
of spectral coordinates in FITS" . This describes the dispersion produced by gratings,
prisms and grisms.
When initially created, the forward transformation of a GrismMap transforms input "
grism parameter" values into output wavelength values. The " grism parameter" is a
dimensionless value which is linearly related to position on the detector. It is
defined in FITS-WCS paper III as " the offset on the detector from the point of
intersection of the camera axis, measured in units of the effective local length" . The
units in which wavelength values are expected or returned is determined by the values
supplied for the GrismWaveR, GrismNRP and GrismG attribute: whatever units are used for
these attributes will also be used for the wavelength values.
" printf" format specifiers identified by " %" symbols in the
normal way. " options" string contains " %" format specifiers,
then an optional list of additional arguments may follow it in order to supply
values to be substituted for these specifiers. The rules for supplying these are
identical to those for the astSet function (and for the C " printf" function). A null Object pointer (AST__NULL) will be returned if this function is invoked with the AST error status set, or if it should fail for any reason.