Page 308 - Modern Optical Engineering The Design of Optical Systems

Chapter
13

Optical System Layout

This chapter will be devoted to the first-order optics of several typical
optical systems. The number of systems covered here is, of necessity,
limited, and the emphasis is placed on those fundamental principles
which are applicable to a broad range of optical systems. The rather
straightforward algebraic manipulations and the considerations of
image size and position which follow are quite typical of those encoun-
tered in the preliminary stages of optical system design. Constructional
details of the optical components have been deliberately omitted and
are discussed at considerable length in later chapters. Note that the
system diagrams in this chapter show the components as simple lenses.
These could equally well be mirrors instead of lenses, and typically are
fairly complex assemblies of lens elements.

13.1 Telescopes, Afocal Systems
The primary function of a telescope is to enlarge the apparent size of
a distant object. This is accomplished by presenting to the eye an
image which subtends a larger angle (from the eye) than does the
object. The magnification, or power, of a telescope is simply the ratio of
the angle subtended by the image to the angle subtended by the
object.* Nominally, a telescope works with both its object and image
located at infinity; it is referred to as an afocal instrument, since it has
no focal length. In the following material, a number of basic relation-
ships for telescopes and afocals will be presented, all based on systems
with both object and image located at infinity. In practice, small

* For large angles, the magnification is the ratio of the tangents of the half-angles.

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