The laser pointer is mounted in two 3D-printed centring rings that grip the cylindrical barrel housing at the front and rear, Fig. . The centring rings can accommodate barrel diameters between 10 and 19 mm (for larger lasers, the centring mounts for the optical cell may be used). To align the laser beam, the thumb-screws on the two rings are manually adjusted until the beam runs along the central axis of the optical cage. We caution that, in our experience, cheap laser pointers are often made with rather loose production tolerances, and the beam axis does not always coincide with the barrel axis. Hence, it is frequently necessary to orient the module at a slight angle to the cage axis in order to achieve the required beam alignment.