A comment here that also applies anywhere this sort of thing is used: these sort of all-caps abbreviations I think are a bad idea.  I know historically they come from situations where bytes were more precious, but here we can make a break with the past and instead use something more readable.  So for example, `spectral_ref_frame` might instead be: ['wavelength', 'frequency', 'energy', 'wave number', 'air wavelength', 'radio velocity', 'optical velocity', 'optical z', 'relativistic v', 'relativistic beta'].  (If these are actually needed to be attributes in some cases, replace the spaces with underscores)