Selection of a particular coordinate system is performed simply by setting a value for the SpecFrame's (character string) System attribute. This setting is most conveniently done when the SpecFrame is created. For example, a SpecFrame representing Energy would be created by:
SPECFRAME = AST_SPECFRAME( 'System=Energy', STATUS )
Note that specifying ``SystemEnergy'' also changes the associated Unit (from metres to Joules). This is because the default value of the SpecFrame's Unit attribute depends on the System attribute setting.
You may change the System value at any time, although this is not
usually needed. The values supported are set out in the attribute's
description in and include a variety
of velocity systems, together with frequency, wavelength, energy,
AST A Library for Handling World Coordinate Systems in Astronomy