This attribute holds the rate of change of the refractive index of the grism material with respect to wavelength at the reference wavelength (given by attribute GrismWaveR). The default value is 0.0 (the appropriate value for a pure grating disperser with no prism). The units of this attribute should be consistent with those of attributes GrismWaveR and GrismG.

Note, the value of this attribute may changed only if the GrismMap has no more than one reference. That is, an error is reported if the GrismMap has been cloned, either by including it within another object such as a CmpMap or FrameSet or by calling the AST_CLONE function.

GrismMap