Optical apparatus includes a mount, which holds a workpiece. An array of optical heads project respective patterns of radiation onto the workpiece. A calibration assembly captures images of the respective patterns. A motion assembly, on which the calibration assembly is mounted, transports the calibration assembly among a plurality of different positions between the array of the optical heads and the mount so as to intercept and image, at each of the different positions, a respective pattern projected by a different one of the optical heads. A processor processes the images captured by the calibration assembly at the different positions so as to monitor operation of the apparatus.
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