## Specifying a Particular Spectral Coordinate System

Selection of a particular coordinate system is performed simply by setting a value for the SpecFrameSpecFrame's (character string) SystemSystem attribute. This setting is most conveniently done when the SpecFrame is created. For example, a SpecFrame representing Energy would be created by:

159#159

Note that specifying “System154#154Energy” 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 Appendix C and include a variety of velocity systems, together with frequency, wavelength, energy, wave-number, etc.