  We devise a systematic method to determine the Fisher information required
for resolving two incoherent point sources with a diffraction-limited linear
imaging device. The resulting Cram\'er-Rao bound gives the lowest variance
achievable for an unbiased estimator. When only intensity in the image plane is
recorded, this bound diverges as the separation between the sources tends to
zero, an effect which has been dubbed as Rayleigh's curse. However, this curse
can be lifted using suitable coherent measurements. In particular, we determine
a class of measurements that can be easily implemented as projections on the
orthogonalised derivatives of the point spread function of the system.
