A wavefront detector (100) and method for determining a signal wavefront (Ws) of a signal beam (Ls). A beam combiner (11) is configured to combine the signal beam (Ls) with a reference beam (Lr). An image detector (12) comprising an array of photosensitive pixels (12p) is configured to receive and measure an interference pattern (Wrs) of the combined signal and reference beams (Lr+Ls). A reference light source (14) is configured to generate the reference beam (Lr). A feedback controller (20) is configured to receive an interference signal (IB) based on measurement of at least part of the combined signal and reference beams (Lr+Ls), and control generation of the reference beam (Lr) by a feedback loop based on the interference signal (IB).