Right, I agree that the parameter should accept the strings... I think maybe what would be better is stating explicitly what each of these strings refer to in the parameter docstring.  I'm just asking that there be a link to the astropy docs.  e.g.:

```
        velocity_convention : 'relativistic', 'radio', or 'optical'
               One of the astropy doppler equivalencies: `~astropy.units.equivalencies.doppler_radio`, `~astropy.units.equivalencies.doppler_optical`, or `~astropy.units.equivalencies.doppler_relativistic`, 
```