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.