Precise photographic monitoring of MEG II thin-film muon stopping target position and shape

Abstract We describe and show results of a photographic technique for continuously monitoring the position, orientation, and shape of a thin-film muon stopping target for the MEG II experiment Baldini et al. (2018). The measurement is complicated by the target being located in a region with 1.3 T magnetic field, significant positron flux, and limited access. The technique achieves a measurement precision of 10 μ m normal to and 30 μ m parallel to the film surface, significantly better than required for the MEG II experiment.