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