This attribute (together with SourceSys, SourceVRF, RefRA and RefDec) defines the " Source" standard of rest (see attribute StdOfRest). This is a rest frame which is moving towards the position given by RefRA and RefDec at a velocity given by SourceVel. A positive value means the source is moving away from the observer. When a new value is assigned to this attribute, the supplied value is assumed to refer to the spectral system specified by the SourceSys attribute. For instance, the SourceVel value may be supplied as a radio velocity, a redshift, a beta factor, etc. Similarly, when the current value of the SourceVel attribute is obtained, the returned value will refer to the spectral system specified by the SourceSys value. If the SourceSys value is changed, any value previously stored for the SourceVel attribute will be changed automatically from the old spectral system to the new spectral system.

When setting a value for SourceVel, the value should be supplied in the rest frame specified by the SourceVRF attribute. Likewise, when getting the value of SourceVel, it will be returned in the rest frame specified by the SourceVRF attribute.

The default SourceVel value is zero.

SpecFrame

• It is important to set an appropriate value for SourceVRF and SourceSys before setting a value for SourceVel. If a new value is later set for SourceVRF or SourceSys, the value stored for SourceVel will simultaneously be changed to the new standard of rest or spectral system.