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.

astGrismMap

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 functions beyond those which are applicable to all Mappings.