  An automatic target monitoring method based on photographs taken by a CMOS
photo-camera has been developed for the MEG II detector. The technique could be
adapted for other fixed-target experiments requiring good knowledge of their
target position to avoid biases and systematic errors in measuring the
trajectories of the outcoming particles. A CMOS-based, high resolution, high
radiation tolerant and high magnetic field resistant photo-camera was mounted
inside the MEG II detector at the Paul Scherrer Institute (Switzerland). MEG II
is used to search for lepton flavour violation in muon decays. The
photogrammetric method's challenges, affecting measurements of low momentum
particles' tracks, are high magnetic field of the spectrometer, high radiation
levels, tight space constraints, and the need to limit the material budget in
the tracking volume. The camera is focused on dot pattern drawn on the thin MEG
II target, about 1 m away from the detector endcaps where the photo-camera is
placed. Target movements and deformations are monitored by comparing images of
the dots taken at various times during the measurement. The images are acquired
with a Raspberry board and analyzed using a custom software. Global alignment
to the spectrometer is guaranteed by corner cubes placed on the target support.
As a result, the target monitoring fulfils the needs of the experiment.
