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In the case of a very distant object, the aid of a more complex optical system — a telescope — must be obtained. Here a lens or mirror of comparatively long focal length is made to produce a real image of the object at its focus, and the image is then magnified by a second lens (eyepiece) as was the object in Fig. 66c. These
are the simple optics of the telescope.
In Fig. 66d, an image of a distant object is formed at I by the objective lens 0. The angular size of the distant object at O (or at the eye, since the object distance is very great) is equal to angle a, which is also the angular size of the image I. This image is magnified by means of an eyepiece, e, to the apparent angular size a'.
It is evident that the linear size of the image depends on the focal length of the objective, and also that the focal length of the eyepiece will determine the size of the angle a’. Thus, the magnifying power of a telescope is given by: M = F/f,
where F is the focal length of the objective, and f is the focal length of the eyepiece.
Although discussions of magnifying power found elsewhere may differ in some respects from the above, the equation last given is the definition of magnifying power which concerns the telescope when used for astronomical purposes.
Field of View. The real (angular) field of view, represented by angle a, Fig. 66d, is measured by that total portion of the image "plane" formed by the objective which can be accepted by the eyepiece. When this field is magnified to the apparent size of angle a', it becomes the apparent (angular) field of view. Evidently, then,
Real field = Apparent field \Magnification
It follows, therefore, that as the magnification is increased the field of view becomes smaller. This effect is aptly illustrated in
Fig. 49.
Fig. 67. A simplified diagram illustrating the reduction of aperture resulting from an exit pupil larger than the pupil of the eye.
Next- Exit Pupil and Eye Relief
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