GrismMap - Transform 1-dimensional coordinates using a grism dispersion equation

**Description:**- 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.

- A GrismMap is a specialised form of Mapping which transforms
1-dimensional coordinates using the spectral dispersion equation
described in FITS-WCS paper III
**Constructor Function:**- astGrismMap
**Inheritance**- The GrismMap class inherits from the Mapping class.

**Attributes**- 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

- GrismNR: The refractive index at the reference wavelength

- In addition to those attributes common to all Mappings, every
GrismMap also has the following attributes:
**Functions**- The GrismMap class does not define any new functions beyond those which are applicable to all Mappings.

Starlink User Note 211

R.F. Warren-Smith & D.S. Berry

7th December 2012

E-mail:starlink@jiscmail.ac.uk

Copyright (C) 2012 Science \& Technology Facilities Council