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Aberrations of the Eyepiece
Only the most prominent of these will be briefly discussed: chromatic aberration, distortion, curvature of the field, and spherical aberration. Chromatic aberration is of two kinds, longitudinal (illustrated in Fig. 4), and lateral. The latter is caused by unequal magnification of the different colors, and is more properly known as chromatic difference of magnification.
The physical effects of the above aberrations (except spherical) can be readily observed by the possessor of a telescope when a simple lens of about 1" focal length is employed for the eyepiece. If one of the lenses is removed from a compound ocular, such an experimental eyepiece is had already mounted.
(For these experiments, the telescope can consist simply of a small achromatic lens of not less than about 6" focal length, taped to one end of a short mailing tube.)
Longitudinal chromatic aberration, uniform in amount throughout the field, is revealed when observing a twig or telephone wire against the bright sky. If you push the eyepiece first inside and then outside of focus, the object will be seen in different colors.
Both positive types of eyepiece (Kellner and Ramsden) have better correction for this fault than does the Huygens. The balance of the experiments may be performed by observations on the straight-edged roof top of a distant building silhouetted against a bright sky.
Chromatic difference of magnification is shown by the increasing amount of color that is seen bordering the roof line as it approaches the edge of the field. Axial images are unaffected, but the error increases in intensity in proportion to the distance from the center of the field. The Huygens eyepiece is practically free of this defect, while there may be a troublesome amount in the outer parts
of the field of the Ramsden. Some color is also present in the Kellner eyepiece.
Distortion shows up on displacing the roof line toward the edge of the field, when its straight edge is seen to become curved, convex toward the center of the field. This is due to unequal magnification of different parts of the image, the aberration increasing as the cube of the distance from the axis. Intolerable in certain terrestrially used telescopes, this aberration is an innocuous one as far as general astronomical observation is concerned. All three compound eyepiece types are substantially free of this error, although more complete correction is found in the Kellner and Ramsden.
Curvature of the field, increasing in amount with the square of the distance from the axis, becomes apparent when the telescope is shifted so that the roof line again divides the field in half; if the center is in sharp focus, curvature of the field makes it necessary to push the eyepiece inward in order to bring the edge into equally
sharp focus. Only partially corrected in the Huygens eyepiece, the defect is almost completely removed from the Kellner and Ramsden.
Spherical aberration (illustrated in Fig. 5) is an insidious error, difficult to detect in a single lens because of the small amount of the aberration present. Where an excessive quantity of spherical aberration is produced, the diffraction pattern (see Chapter XIII) becomes blurred, and contrast suffers. The best obtainable image
may remain apparently unchanged in quality throughout a small but perceptible movement of the eyepiece. In any given eyepiece, the fault varies as the square of the angular aperture of the objective with which the eyepiece is employed. (Angular aperture is the angle, at the focus, subtended by the objective lens or mirror — see Fig. 67.)
For this reason the Huygens, uncorrected for the defect, does not perform well on telescopes of moderate focal ratio; in fact, at f/7 and lesser ratios, it is entirely unsatisfactory. Both the Ramsden and Kellner eyepieces are relatively free of spherical aberration, with the Ramsden surpassing the Kellner in this regard.
Next- Assembling Eyepiece Components
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