  The autocorrelation function and similarly the Fourier-power spectrum of a
rotation measure (RM) map of an extended background radio source can be used to
measure components of the magnetic autocorrelation and power-spectrum tensor
within a foreground Faraday screen. It is possible to reconstruct the full
non-helical part of this tensor in the case of an isotropic magnetic field
distribution statistics. The helical part is only accessible with additional
information; e.g. the knowledge that the fields are force-free. The magnetic
field strength, energy spectrum and autocorrelation length l_B can be obtained
from the non-helical part alone. We demonstrate that l_B can differ
substantially from l_RM, the observationally easily accessible autocorrelation
length of an RM map. In typical astrophysical situation l_RM > l_B. Any RM
study, which does not take this distinction into account, likely underestimates
the magnetic field strength. For power-law magnetic power spectra, and for
patchy magnetic field configurations the central RM autocorrelation function is
shown to have characteristic asymptotic shapes. Ways to constrain the volume
filling factor of a patchy field distribution are discussed. We discuss
strategies to analyse observational data, taking into account - with the help
of a window function - the limited extent of the polarised radio source, the
spatial distribution of the electron density and average magnetic energy
density in the screen, and allowing for noise reducing data weighting. We
briefly discuss the effects of possible observational artefacts, and strategies
to avoid them.
