A SpecFrame is a specialised form of one-dimensional Frame which represents various coordinate systems used to describe positions within an electro-magnetic spectrum. The particular coordinate system to be used is specified by setting the SpecFrame's System attribute (the default is wavelength) qualified, as necessary, by other attributes such as the rest frequency, the standard of rest, the epoch of observation, etc (see the description of the System attribute for details).
By setting a value for thr SpecOrigin attribute, a SpecFrame can be made to represent offsets from a given spectral position, rather than absolute
AST A Library for Handling World Coordinate Systems in Astronomy