Abstract: Gun muzzle deflection detector means comprising a depletion mode linear photodiode sensor having four electric outputs representing elevational and azimuth displacement of an optical signal impinging on the sensor face, said sensor outputs being combined and amplified to provide error signals representative of the muzzle deflection from a true zero deflection condition in the elevation and azimuth directions. An automatic gain control is used to adjust the error signals up or down inversely according to the absolute values of the light impinging on the sensor, thereby compensating for possible errors due to fog or airborne particulates in the optical system.