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It is well known that for a positive powered lens system, the longitudinal magnification is equal to the square of the lateral (or transverse) magnification. It is not immediately obvious that the same relationship would apply in the case of afocal telescopes. Because imagery of close objects through afocal telescopes has received such scant attention in the literature, it was considered that the derivation of this simple longitudinal magnification formula should be presented here.
The following simple arrangement is illustrative of the convenience of this method. The target plate (as a circular button) is attached to the eyecup of a telescope with translucent tape. While the telescope rests on a tabletop, light from a flashlight or other light source is directed through the telescope. The images of the two target surfaces, are located using a shoebox or a piece of card as a screen, which can be moved over graph paper or a meter rule to measure the distance xo between the images. With the reduced thickness xe of the test plate already known, the magnification can easily be determined using Eq. (5).
Apparatus for measuring magnification of Keplerian telescopes. A target plate of thickness
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