This attribute holds the angle (in radians) between the normal to the grating or exit prism face, and the dispersion plane. The dispersion plane is the plane spanned by the incoming and outgoing ray. The default value is 0.0.

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