  We investigate theoretically coherent detection implemented simultaneously on
a set of mutually orthogonal spatial modes in the image plane as a method to
characterize properties of a composite thermal source below the Rayleigh limit.
A general relation between the intensity distribution in the source plane and
the covariance matrix for the complex field amplitudes measured in the image
plane is derived. An algorithm to estimate parameters of a two-dimensional
symmetric binary source is devised and verified using Monte Carlo simulations
to provide super-resolving capability for high ratio of signal to detection
noise (SNR). Specifically, the separation between two point sources can be
meaningfully determined down to $\textrm{SNR}^{-1/2}$ in the units determined
by the spatial spread of the transfer function of the imaging system. The
presented algorithm is shown to make a nearly optimal use of the measured data
in the sub-Rayleigh region.
