docs: def set_obs_angle(self, theta_rad):
          """Set the observer angle relative to the field.
  
          **Call signature**
  
          *theta_rad*
            The angle between the ray path and the local magnetic field,
            in radians.
          Returns
            *self* for convenience in chaining.
          """
          self.in_vals[IN_VAL_THETA] = theta_rad * 180 / np.pi # rad => deg
          return self