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Eyepieces 2
Most prominent of the positive types are the oculars of Ramsden and Kellner. All these eyepieces, for astronomical use, should be mounted in standard-sized tubing of l¼ outside diameter. As to focal lengths, a wide choice is available to the amateur, ranging from about 1½" down to 14" and less.
The field lens of the lowest
power will be about 1" in diameter, as compared to 1/8" or less for the highest powers. Probably the most frequently used eyepiece is one of 1" focal length, and good choices to supplement it are focal lengths of 2/3" and 1/3", although l/2" and 1/4" are more popular. Where a telescope does not have a finder, a low-power eyepiece of about 1½" focal length is a convenient aid in locating objects.
The Huygenian eyepiece (Fig. 68, top) is composed of two plano-convex lenses, of unequal focal lengths, in which the convex surfaces face away from the eye. The field lens is placed inside the focus of the mirror, resulting in a slightly smaller image being formed a little closer to the mirror, the image then being magnified by the eyepiece alone. A diaphragm, excluding all but the useful rays, is placed in the focal plane of the eye lens, about midway between it and the field lens. The well-defined apparent field of view is nearly 40° wide. Because of spherical aberration, the Huygenian does not perform as well as the Ramsden or Kellner types on
moderate- or low-ratio telescopes, but when used at f/10 and above it leaves little to be desired.
In the usual design of the Huygenian, the focal lengths of the field and eye lenses are in a ratio of 3 to 1, with a separation of half the sum of the focal lengths. Thus, in an eyepiece of 1" equivalent focal length, the focal lengths of the field and eye lenses are 2" and 2/3" respectively. Their separation is 11/3", measured, approximately, between the convex surfaces.
Fig. 68. The principal types of astronomical eyepieces. The dotted vertical lines mark the focal planes.
In the Ramsden, or positive ocular (Fig. 68, center), likewise consisting of two plano-convex lenses, the convex surfaces face each other. Well corrected for spherical aberration, this eyepiece performs effectively on all sizes of telescopes, and is preferred to the Huygenian for use on Newtonian reflectors. In its most corrected form, the lenses are of equal focal lengths, and separated by a distance equal to their mutual focal length. But with the lenses so spaced, a scratch or any dust on the field lens is brought into sharp focus and magnified by the eye lens. Nor can a reticle be used with such a combination. Accordingly, it has been found expedient to move the field lens out of focus by bringing the lenses closer together, at the cost of introducing a tolerable amount of color into the outer parts of the field.
Best separations vary from two thirds to three quarters of the focal length of either
lens. The focal plane lies a short distance in front of the field lens, and the eye relief,
somewhat better than in Huygens' eyepiece, is there-by further improved.
Next- Eyepieces part3
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