def dom_positions(self):
        """The positions of the DOMs, calculated from PMT directions."""
        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
The positions of the DOMs, calculated from PMT directions.