This attribute holds the number of grating rulings per unit length. The unit of length used should be consistent with the units used for attributes GrismWaveR and GrismNRP. The default value is 0.0. (the appropriate value for a pure prism disperser with no grating).

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