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Exit Pupil and Eye Relief 2
It is evident that as the magnification is reduced, the exit pupil increases in size. But it should not be permitted to exceed the diameter of the pupillary opening of the eye, for reasons about to be explained. In darkness, the average pupil diameter is between 71/2 and 8 millimeters, as determined by several authorities.
However, in the presence of illumination from a bright star field, some contraction will take place, and a diameter of 7 mm. has been generally accepted as a standard, although it is true that some eyes are able to receive an exit pupil of 7½ mm. or more.
Suppose, for example, an eyepiece of 3" focal length were used with a 6-inch
f/8 mirror, yielding a magnification of 16 and an exit pupil of 9½ mm. As only 7 mm. of this could enter the eye, and as the effective aperture of the mirror is equal to the magnification multiplied by the exit pupil diameter, it is apparent that only about 4½" of the mirror's diameter will be employed. The outer zones are actually
diaphragmed out by the iris of the eye. The net result, indicated by the dotted lines in Fig. 67, is a waste of aperture, accompanied by a loss of illumination.
The value of a 7-mm. exit pupil is that it provides the widest possible field of view commensurate with the proportions of the objective, and is desirable in certain kinds of observations, such as comet seeking. A practical example is the 7 x 50 binocular which is standard equipment in the U. S. Navy. This is the so-called
night glass, most useful in the deepening twilight. It is a 7-power glass, with 50-mm. (2-inch) objectives. The exit pupil is therefore 7 mm. in diameter, quite filling the expanding pupillary opening in the dim light.
When taking the first look through his telescope upon completing the alignment, an amateur may have been troubled by the conspicuous shadow of the obstructing diagonal, especially if a low-power eyepiece were used at the time. Of course, he was pleased, and perhaps mystified, to find that the objectionable shadow was
not apparent at night.
This is because the image of the diagonal, projected on the Ramsden disk, may be fully half as large as the pupillary opening of the eye in broad daylight, whereas it is inconspicuously small by comparison with the greatly expanded pupil at night. Therefore, in daytime or terrestrial observations, high-power eyepieces should be used if this disturbing shadow is to be avoided. Low magnifications can be successfully employed with use of the diaphragm shown in Fig. 87, whereby the central obstruction is eliminated.
The loss of aperture in that case is not a matter of great concern, as there is usually an abundance of illumination in the daytime. Theoretically, the smallest exit pupil which ought to be used is one that will exhaust the full resolving power of the mirror.
This occurs when the magnification is high enough to reveal all the detail in an object, or, in other words, when the diffraction disk becomes visible. This condition is reached with a magnification of about 13 per inch of aperture, when the exit pupil becomes about 2 mm. in diameter. Practically, however, as stated in Chapter XIII,
in the section Resolving Power, it will frequently be necessary to resort to considerably higher magnifications.
Next- Eyepieces
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