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.

AST_GRISMMAP

The GrismMap class inherits from the Mapping class.

In addition to those attributes common to all Mappings, every GrismMap also has the following attributes:

• GrismNR: The refractive index at the reference wavelength

• GrismNRP: Rate of change of refractive index with wavelength

• GrismWaveR: The reference wavelength

• GrismAlpha: The angle of incidence of the incoming light

• GrismG: The grating ruling density

• GrismM: The interference order

• GrismEps: The angle between the normal and the dispersion plane

• GrismTheta: Angle between normal to detector plane and reference ray

The GrismMap class does not define any new routines beyond those which are applicable to all Mappings.