A lithographic apparatus includes an alignment sensor configured to determine the position of an alignment target having a periodic structure. The alignment sensor includes a demultiplexer to demultiplex a number of intensity channels. The demultiplexer includes a number of stages arranged in series and a number of demultiplexing components, each demultiplexing component operable to divide an input radiation beam into two radiation beam portions. The first stage has a first demultiplexing component that is arranged to receive as an input radiation beam an incident radiation beam. Each successive stage is arranged such that it has twice the number of demultiplexing components as a preceding stage, each demultiplexing component of each stage after the first stage receiving as an input one of the radiation beam portions output from a demultiplexing component of the preceding stage.