  The Rasnik system is a 3-point optical displacement monitor with
sub-nanometer precision. The CCD-Rasnik alignment system was developed in 1993
for the monitoring of the alignment of the muon chambers of the ATLAS Muon
Spectrometer at CERN. Since then, the development has continued as new CMOS
imaging pixel chips became available. The system's processes and parameters
that limit the precision have been studied in detail. We conclude that only the
quantum fluctuations to which the light level content of sensor pixels are
subject to, is limiting the spatial resolution. The results of two Rasnik
systems are compared to results from simulations, which are in good agreement:
the best reached precision of $\SI{7}{pm/\sqrt{Hz}}$ is reported. Finally, some
applications of high-precision Rasnik systems are set out.
