An image converter tube 2c and a plurality of image sensors CCDs 1 are provided, and the respective CCDs 1 and image positions in the image converter tube 2c are in one-to-one correspondence. At said instant, a phase shift gradient is also estimated, associated with a shift in frequency, and a frequency correction is applied, said correction being applied and maintained for the duration of a subsequent symbol. In addition, horizontal positions of each of the arms on the cross plate can be independently adjusted using the brackets.