the method for positioning an array with periodic structures , for forming an image thereon , comprises the following steps : providing a first beam of light by a first light source ; passing this first beam of light through the array ; forming a light line corresponding to the first light source with the first beam of light after the first beam of light passes through the array such that the light line is perpendicular to long axes of the periodic structures ; rotating the array to rotate the light line across a first detector ; producing a signal when the light line sweeps across the first detector ; determining when maximum signal is produced by the first detector ; and rotating the array to a proper position based on angular position corresponding to this maximum signal . according to an another embodiment of the present invention , an apparatus for locating an array having a periodic structure comprises : a stage rotatably supporting the array ; a first light source , providing a first alignment light beam , the first light source being located to project light onto one side of the array ; a coarse alignment detector located on another side of the array ; and an alignment lens located between the array and the course alignment detector , the alignment lens and the array , together , providing a light line corresponding to the first light source in a plane where the course alignment detector is located , wherein rotation of said array produces a corresponding rotation of the light line in the plane .