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Eyepieces
The apparent field of a single convex lens used as an eyepiece is considerably smaller than assumed in Fig. 66d. It can be seen in that diagram that unless the angle a is small, the field rays from the edges of the objective, after passing through the edges of the image plane, will miss that eyepiece altogether. And, because of inherent aberrations, the sharply defined portion of the visible field will be further reduced, extending not more than 5° on either side of the axis.
It was chiefly to overcome this defect of the single lens that the compound or two-lens ocular was devised. For example, by placing a convex field lens in the vicinity of I (Fig. 66), the divergent rays referred to above can be collected and converged and made to enter the eye lens; thus a considerably larger field of view can be encompassed.
This is shown to advantage in Fig. 65, the Ramsden eyepiece there having an apparent field of 36°. Placed so close to the focal plane, the field lens contributes but little to the magnification, most of which is accomplished by the eye lens, but by a suitable choice of the focal lengths and separation of the components, aberrations can be materially reduced. The equivalent focal length (e. f. I.) of a combination of two lenses when thus used is equal to that of a single lens that will yield the same magnification, and is:
f1f2
e.f.l.=
f1 = f2 – d’
where d is the separation of the lenses.
The linear size of the real field of view can then be taken as approximately the same as the diameter of the field lens of the eyepiece. For an equivalent focal length of 1", the clear aperture of the field lens may be about 0.7"; a field of this width at the 48" focus of a mirror will have an angular size of about 50 minutes
of arc.
As the magnification realized in this instance will be 48 (F/f), the field will be enlarged to an apparent diameter of 40°; a field of this size is frequently attained in a two-lens eyepiece. Broadly speaking, astronomical eyepieces or oculars can be
classified into two types, negative and positive. In the negative type the image is formed between the lenses, or within them, thereby precluding the use of this eyepiece as a simple magnifier. The negative type is represented chiefly by the eyepiece devised by Huygens.
Next- Eyepieces part2
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