The focal length of a telescope or camera lens is a parameter that determines the size of objects in the image (Figure 5.8). In conjunction with the film or sensor size, the focal length also determines the field of view.2
2 If you're new to optical diagrams, you may wonder why Figure 5.8 shows rays of light spreading apart while Figure 5.1 shows them coming together. The answer is that Figure 5.1 shows two rays from the same point on a distant object, but Figure 5.8 shows one ray from each of two points some distance apart.
If your telescope is a refractor or Newtonian, the focal length is also the length of the tube (the distance from the lens or mirror to the sensor or film). Technically, the focal length is the distance at which a simple lens forms an image of an infinitely distant object, such as a star.
Telescopes in the Cassegrain family have a focal length longer than the actual tube length because the secondary mirror enlarges the image. For example, the popular 8-inch (20-cm) f /10 Schmidt-Cassegrain packs a 2000-mm focal length into a telescope tube about 500 mm long.
The way a camera is coupled to a telescope affects the focal length of the resulting system (Table 5.1). With direct coupling, you use the telescope at its inherent focal length, unmodified. Positive projection and afocal coupling usually increase the focal length; negative projection always does. Compression always reduces it.
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