The complexity of optical systems depends on the specifications. Each design therefore requires a minimum number of lenses. The optical designer must exceed the specification values for two reasons: first to allow reasonable manufacturing and material tolerances and, second, to ensure that the imaging quality remains constant under environmental changes like temperature and air pressure fluctuations during the whole lifetime of the instrument.
On the other side, the design should have as few lenses as possible to minimize costs but also not be forced to spread the available tolerance budget on too many lenses.
We see that designing and tolerancing are trade-off processes, which require a lot of experience for the designer, good communication with the production engineers, and many iteration loops. Nevertheless, new parameters are welcome to relax the situation. This is part of the assembly concept, where by moving or tilting selected lenses in all three directions, one tries to get the system into the final specifications. In Secs. 8.2 and 8.4, we will call these adjustment parameters "compensators" to express their main function, that is, to compensate the residual errors of component production and of material uncertainties.
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