An optical receiving device for receiving an optical signal (60) from a signal collector (50) comprising a detector (40) for receiving the optical signal (60) form the collector (50) and a signal mask (45) having an aperture (47) through which at least a portion of the signal (65) passes, said mask (45) located along an optical axis (66) of the signal intermediate the collector (50) and the detector (40) wherein the location of the signal mask (45) is such so as the attenuate the signal (65) to less than a saturation threshold of the detector (40) and a conjugate image plane (70a) of the signal (65) is located within a range form coincident with the mask (45) to intermediate the mask (45) and detector (40).
