Lower performance optical instruments and ones to be mass-produced are generally assembled by the "drop-in" technique described in Section 4.2. Essentially, one puts the optics in place, secures them by some means, and accepts whatever performance results. If higher performance is needed, one might tighten the tolerances on optics andâor mechanical parts. Other instrument designs keep the tolerances relatively loose and adjust alignment to improve performance. The highest performance instruments, such as optical projection systems for microlithography, set the tolerances to the highest level feasible and fine-tune alignment of carefully selected elements to achieve the maximum possible performance.
The premise upon which this chapter is written is that adjusting the alignment of an optic or system of optics and then securing that alignment is a valid method for boosting performance. Naturally, a trade-off exists between the costs of tight tolerances and the costs of adjustment mechanisms, tools, fixtures, and labor. To assist in making this choice, here we consider various aspects of alignment technology. We deal first with techniques that may be used to align individual optical components to their mounts. That discussion relates closely to and extends the discussion of centering techniques given in Section 2.1.2. We then consider alignment of systems comprising lenses, mirrors, and combinations thereof. Space constraints prohibit an exhaustive treatment of these subjects. Instead, we summarize some techniques that have proven useful. Most of these have been described in the literature. References are given wherever possible so the reader can find more details about topics of interest.
There are two closely related aspects of alignment: measuring errors and using mechanisms of some sorts to reduce those errors to acceptable magnitudes. We therefore consider both of these aspects for each technique considered. A final topic receiving special attention is the creation of prealigned modules needing no further adjustment when installed into an instrument. At the single-lens level, the module is sometimes referred to as a "poker chip." When two or more lenses are involved, we have a modular subassembly. Modular design reduces the time and effort required at assembly. It is most frequently applied in cases where the increased design, tooling, and fixturing costs of modularization can be amortized over large quantity production.
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