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This is a known type of system in which a suitable rotating element in the optical system causes the image of the star field to nutate over the surface of a mask divided into alternative sectors of opaque and transparent material. When the lock star is on the axis of the entry optical system, the lock-star image traverses a circular path on the mask concentric with the mask giving rise to an unvarying frequency in the output of a photo cell behind the mask. With misalignment of the star image from the axis of the entrance optical system, the circular track is displaced as the result of which the output of the photo cell is frequency modulated. The resulting FM wave is suitably processed to provide misalignment signals along two axes. This paper derives the value of Â¿ = (r/R)(S/N)-1/2 for the system, using various photomultiplier tubes used as photo cells, and various parameters such as angular velocity of star image, number of sectors on mask, and diameter of objective of entrance optical systems: r = radial displacement of star-image track. R = radius of star-image track.