code: def dom_positions(self):
          
          if not self._dom_positions:
              for dom_id in self.dom_ids:
                  mask = self.pmts.dom_id == dom_id
                  pmt_pos = self.pmts[mask].pos
                  pmt_dir = self.pmts[mask].dir
                  centre = intersect_3d(pmt_pos, pmt_pos - pmt_dir * 10)
                  self._dom_positions[dom_id] = centre
          return self._dom_positions
docs: The positions of the DOMs, calculated from PMT directions.