Abstract
Several methods have been developed through the years to mount lenses with the ultimate goal of minimizing their positioning errors with respect to the nominal optical layout. This is a non-trivial task, since it requires either very well controlled manufacturing tolerances or alignment of the optical components into their mounts. This paper reviews the classical lens mounting methods and introduces new solutions to improve the centering accuracy. First, an improved drop-in method called auto-centering is described. This method is based on the use of geometrical relationship between the lens diameter, the lens radius of curvature, and the thread angle of the retaining ring to provide centring error typically less than 0.5 arcmin. In addition, an innovative method that relies on geometric principles to auto-center optomechanical parts to each other is described. The method allows to auto-center an optical group in a main barrel, to perform an axial adjustment of an optical group inside a main barrel, and to perform stacking of multiple barrels within 5 µm of centering error. Finally, to improve the centering accuracy of common method used to center injection molded plastic lenses, a new concept using toroidal interfaces has been developed. This method allows a reduction by at least a factor of two the lens centering error compared to methods based on radial clearance fit for the same manufacturing tolerances.
Highlights
In most common lens mounting methods, the lens is dropped into a barrel and secured in place with a retainer or an adhesive [1]
There are manufacturing variations associated with the manufacturing processes such as die machining and molding
Centering error between the two optical surfaces of a single lens, lens thickness error, and molding distortions are intrinsic to the manufacturing process and are not compensated by the mounting interfaces
Summary
In most common lens mounting methods, the lens is dropped into a barrel and secured in place with a retainer or an adhesive [1]. The positioning error of the lens is controlled by manufacturing tolerances without any alignment With this mounting method called drop-in assembly, several parameters influence the lens positioning accuracy. When the centering requirements are at the tolerancemanufacturing limit for the drop-in method, other strategies to align the lens need to be considered. Active alignment techniques such as aligning a lens directly in a barrel, sub-cell assembly, or alignment in 5 degrees of freedom with sophisticated equipment are some of the solutions envisioned. All these different implementations of the active alignment provide high-centring accuracy, and. Require expensive equipment and more assembly steps to perform the alignment, resulting in a cost increase
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