The Finder 2
So that the finder can be aligned axially with the main telescope, it is usual to suspend it inside of two rings, and clamp it there by means of screws, in a manner similar to that in which the shafting is shown centered in the fitting in Fig. 80. The rings can be cut from large-diameter tubing, and posts for attaching to the main telescope soldered to them. The minimum height of the finder's axis above
the telescope tube should be about 2".
While the finder is a convenience, it is by no means an essential accessory on a
telescope of the size we are making, especially if setting circles are to be attached
to the mounting. But if the instrument is to be portable, circles will have little practical value, and should be omitted.
Then the telescope may be aimed as though it were a gun, at the approximate spot in the heavens where it is desired to observe; a brief search in that vicinity will usually bring the object sought into the field of a low-power eyepiece. A pair of sights, such as axially aligned protruberances on the tube, may facilitate the above procedure.
However, many observers will appreciate the usefulness of a finder, and although it
is desirable to encumber a portable telescope as little as possible, the added
weight of a small refractor need not be objectionable. Fig. 65. Plan (about 4/9 actual size) of image formation in telescope and eye. The refractor consists of a 1.7" objective, focal length 9", and a Ramsden eyepiece of 1½" focal length, giving an apparent field of 36°. Magnification is 6x; the exit pupil is 7 mm. in diameter; and the real field of view is 6°. The latter is rather more than is desired in a finder, and for this purpose the focal plane (pf) might be stopped down to about half its present size, or an eyepiece of shorter focal length can be substituted.
Chapter 9 Eyepieces and Related Problems
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