This specifies the origin from
which all spectral values are measured. The default value (zero) results in the
SpecFrame describing
absolute spectral values in the system given by the
System attribute (e.g. frequency, velocity, etc). If a
SpecFrame is to be used to describe offset from some origin, the SpecOrigin attribute should be set to
hold the required origin value. The SpecOrigin value stored inside the SpecFrame structure is
modified whenever SpecFrame attribute values are changed so that it refers to the original spectral
position.
When setting a new value for this attribute, the supplied value is assumed to be in the system, units
and standard of rest described by the SpecFrame. Likewise, when getting the value of this
attribute, the value is returned in the system, units and standard of rest described by the
SpecFrame. If any of these attributes are changed, then any previously stored SpecOrigin
value will also be changed so that refers to the new system, units or standard of rest.