Embodiments of the present invention provide a method of estimating a magnitude of background radiation for each of a plurality of regions of a target object comprising providing an estimate of background radiation detected by a detector, measuring radiation scattered by the target object at the detector for each of a plurality of positions of the object with respect to the incident radiation, calculating, for each of the positions, an estimate of a wavefront at the detector, and determining, for each position, an estimated wavefront comprising a coherent contribution from radiation scattered by the target object and a background contribution, wherein said background contribution is at least partly incoherent with the radiation scattered by the target object. This method is particularly suitable for performing coherent diffractive imaging using ptychography where contribution from the incoherently scattered background is taken into account.