1. Apparatus for producing a signal representing the position of a reference pattern in a pattern field, the apparatus comprising, a plurality of correlator stores each having a plurality of storage locations for storing respective pieces of data address inputs for receiving address signals defining addresses for selecting the locations, and an output for producing data stored at any of the locations selected by the address signals, the stores being adapted to store respective pieces of data of predetermined value at locations having reference addresses defined in a predetermined manner by the reference pattern, means for successively presenting to the address inputs of the correlator stores sets of address signals defining addresses defined in the said predetermined manner by successive portions of the pattern field, respective ones of the address signals of each set being presented to the address inputs simultaneously to produce at the said outputs the data stored at the locations selected by the addresses, means coupled to the said outputs to produce correlation scores indicative of the amount of predetermined data selected by the addresses, means for indicating the position relative to the pattern field of the portion of the pattern field defining the addresses presented to the correlator stores, and means for determining the position indicated by the indicating means which is associated with the best correlation score produced by the correlation score producing means and for producing from the determination the said signal representing the position of the reference pattern in the pattern field.