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Typically, the goal of the optical design is to meet the image quality, size,weight, and cost requirements over the desired waveband, aperture, and FOV. It is also very important to consider the system stray light requirements and choose an optical design form that will meet them. This hapter shows that not all design forms have equal stray light performance, and some stray light requirements simply cannot be met with some optical design forms, regardless of how many black surface treatments and baffles are added to the system. This chapter discusses methods for controlling stray light in the optical design form, and the pros and cons associated with each. Because it is often difficult to say whether or not one method is better than another (all of them involve trade-offs), they are presented here in no particular order. Methods used to control external stray light are also useful in controlling internal stray light, and therefore the applicability of each method to both external and internal stray light control is discussed. Nonuniformity compensation and reflective warm shields, which are used to internal stray light only, are discussed. Not all of the methods discussed are appropriate for all systems; for instance, space constraints may make it impossible to add a field stop. Determining which methods are appropriate requires balancing the system stray light requirement with all other requirements.
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