In this chapter we will be considering the design of eyepieces and afocal systems for the visual waveband. The most common afocal system is a telescope. Since the major problem in designing visual afocal systems is the eyepiece, these subjects are naturally connected, and it is logical to consider them together.
In most instruments, eyepieces are just one part of a more complex optical system, but, as we will see, it is usually sensible to consider the design of the eyepiece more or less independently of the rest of the system.
Figure 11.1 shows a complete afocal system. Following the normal convention with light travelling from left to right, this diagram shows the complete system the ârightâ way round. The aperture stop is at the objective lens, the first element on the left. The real image of the objective is the exit pupil of the system, shown as the last surface on the right. After the objective is a block of material that represents a series of prisms that invert the image and present an upright image to the observer. To the right of the prism assembly is the eyepiece lens, which will be the subject of the first section of this chapter.
Although no physical element is located at the exit pupil, when the eye of the observer is located there, he or she will see a bright, magnified image. If, because the user wears spectacles or for any other reason, the pupil of the eye cannot be located at and centered on the exit pupil, the field of view may be reduced, or the image could vanish entirely. The distance between the last surface of the eyepiece and the exit pupil is called the eye relief.
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