  We present a Bayesian maximum likelihood analysis of Faraday rotation measure
(RM) maps of extended radio sources to determine magnetic field power spectra
in clusters of galaxies. Using this approach, it is possible to determine the
uncertainties in the measurements. We apply this approach to the RM map of
Hydra A and derive the power spectrum of the cluster magnetic field. For Hydra
A, we measure a spectral index of -5/3 over at least one order of magnitude
implying Kolmogorov type turbulence. We find a dominant scale of about 3 kpc on
which the magnetic power is concentrated, since the magnetic autocorrelation
length is lambda_B = 3 +/- 0.5 kpc. Furthermore, we investigate the influences
of the assumption about the sampling volume (described by a window function) on
the magnetic power spectrum. The central magnetic field strength was determined
to be about 7 +/- 2 muG for the most likely geometries.
