This specifies the origin from which all spectral values are measured.
The default value (zero) results in the SpecFrameSpecFrame describing
absolute spectral values in the system given by the SystemSystem 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.