When an SpecMap is first created, it simply performs a unit (null) Mapping. Using the AST_SPECADD routine, a series of coordinate conversion steps may then be added. This allows multi-step conversions between a variety of spectral coordinate systems to be assembled out of a set of building blocks.
Conversions are available to transform between standards of rest. Such conversions need to know the source position as an RA and DEC. This information can be supplied in the form of parameters for the relevant conversions, in which case the SpecMap is 1-dimensional, simply transforming the spectral axis values. This means that the same source position will always be used by the SpecMap. However, this may not be appropriate for an accurate description of a 3-D spectral cube, where changes of spatial position can produce significant changes in the Doppler shift introduced when transforming between standards of rest. For this situation, a 3-dimensional SpecMap can be created in which axes 2 and 3 correspond to the source RA and DEC The SpecMap simply copies values for axes 2 and 3 from input to output), but modifies axis 1 values (the spectral axis) appropriately.
For details of the individual coordinate conversions available, see the description of the AST_SPECADD routine.
AST A Library for Handling World Coordinate Systems in Astronomy