What about Solar System or other data which cannot be described at all by RA and Dec (because the body moves too fast, for example?) For example, many ALMA observation sets will contain a moon as amplitude calibrator along with extragalactic sources. It would be nice to have an option e.g. 'coosys other' if not all the options. Alternatively, one could allow RA and Dec to be nil and identify Solar System objects by name and time-date.