Precision Alignment Using a System of Large Rectangular Fresnel Lenses

A system of large rectangular Fresnel lenses has been used in the laser alignment system for the SLAC two-mile accelerator. The alignment system consists of a He-Ne laser light source, a photoelectric detector, and the lenses, one of which is located at each of 297 points which are to be aligned. Each lens has the proper focal length to focus the laser to a point image at the detector. When the alignment at a certain point is to be checked, the image from that lens is scanned by the detector. The image is found to be displaced from its normal position by an amount equal to the product of the position error and the magnification of the lens. The alignment sensitivity is +/-0.0025 mm. The targets are enclosed in a 60-cm diam evacuated light pipe to avoid atmospheric disturbances.