The Finder
Usually attached to the reflecting telescope is a small refractor, of short focal length and low power and possessing a wide angular field of view. For a 6-inch telescope, the finder may be anywhere from 1" to 2" in aperture, and 6" to 12" in focal
length. When it is aligned axially with the mirror, the sighting of celestial objects is made easy by reason of the finder's large field.
Crosshairs or wires are usually stretched diametrically in front of the field lens of the finder eyepiece, exactly in the focal plane of the objective and intersecting at right angles in the center of the field of view. When the object sought has been brought to
the intersection of the crosshairs in the finder, it will then also be in the center of the field of view of the mirror. Instead of crosshairs, a reticle, consisting of a thin glass disk on which cross lines have been etched, may be more simply installed. Fig. 65 is representative of the small telescope that is used.
From the stores of salvaged war surplus goods, achromatic objective lenses of 1" to 2" aperture and 6" to 12" focal length can be picked up at very low cost, as can the lenses for the finder eyepiece. More simply, though, as there is little need to be critical of aberrations, the objective lens might be an ordinary convex spectacle
lens and the eyepiece a plano-convex lens of 1" to 2" focal length.
But as a field at least 3° or 4° in diameter is desirable, a field lens (see discussion in next chapter) of similar focal length should be placed in or near to the focal plane of the objective lens. Means of focusing the eyepiece can be dispensed with, the lenses being secured in a single tube, a la Galileo, with the eye lens
fixed at best focus for a star.
Perhaps the most suitable location for a finder is that occupied on the 8-inch telescope in Fig. 96, where the two eyepieces are in proximity. Often a star diagonal (right-angle prism) is incorporated in the body of the finder, deflecting the optical axis of its objective lens at right angles, so that both eyepieces may be brought adjacent to each other. In this way the observer looks in the same direction in using either instrument, but opinion seems to have it that it is preferable for the purpose of finding to look in the direction of the object.
(In the telescope in Fig. 96, the weight of the eyepiece holder and the finder, both made of heavy bronze parts, necessitated the addition of the counterweight on the opposite side of the tube, to restore the center of gravity to the tube's axis. A sliding adjustment of the weight was provided to compensate for slight error in locating the balance about the declination axis.)
Next- The Finder part2
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