@observer.setter
def observer(self, value):
    if (self.observer is not None):
        raise ValueError('observer has already been set')
    self._observer = self._validate_coordinate(value, label='observer')
    if (self._target is not None):
        self._radial_velocity = None
@observer.setter
def observer(self, value):
    if (self.observer is not None):
        raise ValueError('observer has already been set')
    self._observer = self._validate_coordinate(value, label='observer')
    if (self._target is not None):
        self._radial_velocity = None