An SpecMap is a specialised form of Mapping which can be used to represent a sequence of conversions between standard spectral coordinate systems. This includes conversions between frequency, wavelength, and various forms of velocity, as well as conversions between different standards of rest.
When a 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, selected from the list of supported conversions. This allows multi-step conversions between a variety of spectral coordinate systems to be assembled out of the building blocks provided by this class.
For details of the individual coordinate conversions available, see the description of the AST_SPECADD routine.
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).
" RA" and " DEC" arguments for the individual conversions listed in
description of the " SpecAdd" routine. The nature and units of the coordinate values supplied for the first input (i.e. the spectral input) of a
SpecMap must be appropriate to the first conversion step applied by the SpecMap. For instance, if the
first conversion step is " FRTOVL" (frequency to relativistic velocity), then the coordinate values for
the first input should be frequency in units of Hz. Similarly, the nature and units of the coordinate
values returned by a SpecMap will be determined by the last conversion step applied by the SpecMap.
For instance, if the last conversion step is " VLTOVO" (relativistic velocity to optical velocity), then the
coordinate values for the first output will be optical velocity in units of metres per second. See the
description of the AST_SPECADD routine for the units expected and returned by each
conversion.
A null Object pointer (AST__NULL) will be returned if this function is invoked with STATUS set to an error value, or if it should fail for any reason.