Two-element systems have a wide variety of arrangements and
applications. The following selected examples show basic systems with two lens elements without addressing design details (necessary to correct aberrations) of the components.
Telescopes served in Chapter 5 to demonstrate and explain the vignetting effect. Here we want to identify them as instruments used to observe the image of a distant object, formed by a lens (the telescope objective), and viewed with a second element (the eyelens, also called the ocular). Figure 6.1 shows the basic arrangement of the so-called Kepler telescope, also known as the astronomical type. The image to be viewed is upside down, which is of no concern for astronomical observations. If the device is used for terrestrial applications, a relay lens is inserted between the objective and the eyepiece to render the image orientation upright, which indicates, of course, that it is a three-element system.
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