Page 309 - Modern Optical Engineering The Design of Optical Systems

288 Chapter Thirteen

departures from these infinite conjugates are the rule, but for the most
part they may be neglected. However, the reader should be aware that
the fact that the object and/or the image are not at infinity will occa-
sionally have a noticeable effect and must then be taken into account.
This is usually important only with low-power devices. See also the
comments on instrument myopia in Sec. 8.4.
There are three major types of telescopes: astronomical (or inverting),
terrestrial (or erecting), and Galilean. An astronomical or Keplerian
telescope is composed of two positive (i.e., converging) components
spaced so that the second focal point of the first component coincides
with the first focal point of the second, as shown in Fig. 13.1a. The
objective lens (the component nearer the object) forms an inverted
image at its focal point; the eyelens then reimages the object at infinity
where it may be comfortably viewed by a relaxed eye. Since the internal
image is inverted, and the eyelens does not reinvert the image, the
view presented to the eye is inverted top to bottom and reversed left to
right.
In a Galilean, or “Dutch,” telescope, 13.1b, the positive eyelens is
replaced by a negative (diverging) eyelens; the spacing is the same, in
that the focal points of objective and eyelens coincide. In the Galilean
scope, however, the internal image is never actually formed; the object

Figure 13.1 The three basic types of telescope.