14 Optical focusing-error detection device, An optical focusing error-detection device is described which comprises a radiation sensitive detector which comprises four sub-detectors, and an astigmatic lens system arranged between the detector and the objective system whose focusing error is to be detected. If the astigmatic system comprises two cylindrical lenses having mutually perpendicular axes and having different powers, which lenses are arranged in a non-collimated beam, the resulting focusing-error signal will be linear around the zero point and will also be symmetrical.
OPTICAL FOCUSSING-ERROR DETECTION ARRAY