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Assembling Eyepiece Components
The making of small lenses is a craft that will not be discussed here, as there is already considerable literature on the subject, available to those who would
like to experiment in that direction.2 It might be mentioned, however, that a spindle and lathe are essential for eyepiece manufacture.
Eyepiece making is a fascinating hobby. If one is talented in working to small dimensions, he can, with a little practice and the selection of good glass, succeed in producing as good eyepieces as can be obtained anywhere.
An excellent eyepiece can be assembled from a suitable choice of plano-convex lenses. The focal length of a plano-convex lens is equal to twice the radius of curvature of the convex surface, but as this will very likely be unknown, the focal length can be found by measuring the distance from the lens to the bright image of the sun which is formed by the lens. Or stand it between an illuminated pinhole perforation and a screen. When arranged so that a sharply focused image of the pinhole falls on the screen with the lens exactly 2" midway, the focal length is one half either distance. Or, at any distance, the formula for conjugate foci can he applied: where F is the focal length, Do and Di the distances to pinhole and
image.
Measurements are made from the convex surfaces. The formula for the equivalent focal length of two lenses has already been given. An excellent adjunct to experimentation with small lenses is plasticine or sculptors' modeling clay. A wad of such material stuck on a board will hold the lenses securely fixed, and they can
be manipulated at will.
Next- Chapter 10 The Mounting
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