Chapter 9 Eyepieces and Related Problems
Nearly every optical principle as applied to a telescope requires consideration of the function of the eyepiece when the instrument is used visually. Most first telescopes are built for visual observing, so a knowledge of eyepieces is essential if the best possible results are to be obtained from the instrument as a whole.
As will be seen in this chapter, there are good and poor eyepieces, and some
good ones are not applicable to every type and size of telescope; be very sure, therefore, to equip your instrument with suitable eyepieces of good quality.
Magnification. In general, magnification consists of increasing the visual angular size of an object, which increase can be accomplished by reducing the distance between the eye and the object, either actually or apparently. In Fig. 66a, the distant object D subtends an angular size, a, at the eye. When its distance is reduced by half, as at D', thereby doubling its angular size at the eye, the object appears twice as large (linearly) as before. And so, by means of further angular enlargement, an increasing amount of resolvable detail in the object is made visible.
But there is a physiological limit to which the distance from eye to object can be reduced. For a normal eye, 10" is accepted as being the distance of most distinct vision; any lesser distance imposes an undue strain on the eye muscles. In Fig. 66b, an object, 0, removed about 10" from the eye, subtends there the angle a. But,
the better to define small detail, it is desired to bring the object to within an inch of the eye, at which relative distance its angular size will be increased up to 10 times (for small angles). The eye, however, although strained to the utmost, cannot accommodate for so small a distance, and the object appears blurred.
Therefore, to relieve the strain, a convex lens is introduced (Fig. 66c) to produce a virtual image of the object about 10" in front of the eye, where the image can be examined in comfort. To meet these conditions, the lens in this case must have a focal length of 1"; the image 0’ will then be, apparently, 10 times the size of the object 0, or as the ratio of angle a' to a. It follows, therefore, that the magnifying power of any single lens or combination of lenses used as a simple microscope is given by: M = 10/f, where f, in inches, is the focal length of the lens, or the equivalent focal length of a set of lenses.
Next- Field of View
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