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Eyepieces 3
Specifications for the generally approved Ramsden designs, for equivalent focal lengths of 1", are as follows: (1) focal length of each lens, 1 1/3", separation 0.89"; (2) focal length of each lens, 1 1/4", separation 0.93". The lesser spacing affords greater eye relief, and so is preferred for high powers. The good, sharp field of
either arrangement is about 30° in extent, but fields from 35° to 40° are usually provided.
A modification of the Ramsden ocular, in which the eye lens is formed of an achromatic combination of crown and flint glass, is called the Kellner (Fig. 68, bottom). This eyepiece gives a flat, almost colorless field about 50° in diameter, about as large a field as can ordinarily be used. It is highly recommended in low powers, where its wide field can be used to advantage in variable star work, comet seeking, and so forth.
Specifications, for an equivalent focal length of 1", are: focal length of field lens, 1¾"; focal length of eye lens, 1 1/3"; separation ¾". Troublesome "ghost" images are sometimes present with both the Kellner and Ramsden oculars, arising from reflections originating at the convex surface of the field lens and coming to a focus close to the focal plane of the eye lens. These can be materially subdued by coating both field-lens surfaces with an antireflection fluoride, at the same time improving the image brightness. Indeed, so marked is the improvement in the performance of fluoride-coated optical elements that uncoated lenses are as outmoded as speculum mirrors.
In a general way, this summarizes the characteristics of the more commonly used astronomical eyepieces. Slight variations from the specifications given are frequently resorted to in order to reduce one or another of the several aberrations. But the amateur must be wary of eyepieces not known to come from a reliable manufacturer of optical goods. A good eyepiece is not cheap — it may cost six
or seven dollars or more. Occasionally, an unscrupulous dealer will select from stock an ill-matched pair of lenses that will yield an image, mount them in a cell, and offer them for sale. Such eyepieces are usable, but image formation may suffer considerably, and no reliance can be placed on the assigned focal length.
Next- Aberrations of the Eyepiece
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