Abstract: A photon signal received on a CCD is digitized and coupled to a 3.times.3 pixels data analysis array (DAA). The digital signals read out from the penultimate three pixels of the DAA are used to address a first look up table held in a memory, the digital signals read out from the last three pixels of the DAA are used to address a second look up table held in a memory. The data stored at the two addressed locations are then read out and logically ANDed to produce an output signal only when both ANDed data correspond to a valid photon event. The output signal thus defines the position of the center of the photon signal at a high level of precision.