Lithographic apparatus and alignment method

A lithographic apparatus comprising a source collector module including a collector, configured to collect radiation from a radiation source; an illuminator configured to condition the radiation collected by the collector and to provide a radiation beam; and a detector arrangement comprising a reflector arrangement disposed in a fixed positional relationship with respect to the illuminator, the reflector arrangement being arranged to reflect radiation from the source collector module; and a sensor arrangement disposed in a fixed positional relationship with respect to the reflector arrangement, the sensor arrangement being configured to measure at least one property of radiation reflected by the reflector, the detector arrangement being configured to determine the location of a far field position of the radiation as a function of at least one property of the radiation reflected by the reflector and measured by the sensor arrangement.