In order to get a laser beam to go along any line (indicated by the two pinholes PH1 and PH2), it is possible to use two manually steerable mirrors (M1 and M2) and follow the procedure illustrated here: (a) with M1, centre the beam on PH1; (b) with M2, centre the beam on PH2; repeat the previous two steps iteratively. The convergence of this procedure is greatly improved by placing PH1 as close as possible to M2 and PH2 as far away as possible from M2 .
Fig. 8.9 — Beam alignment technique