Page 158 - Modern Optical Engineering The Design of Optical Systems

Prism and Mirror Systems 141

7.11 Erecting Prism Systems
In an ordinary telescope, the objective lens forms an inverted image
of the object, which is then viewed through the eyepiece. The image
seen by the eye is upside down and reversed from left to right, as indi-
cated in Fig. 7.23. To eliminate the inconvenience of viewing an
inverted image, an erecting system is often provided to re-invert the
image to its proper orientation. This may be a lens system or a prism
system.

Porro prism of the first type
The most commonly used prism-erecting system is the Porro prism of
the first type, illustrated in Fig. 7.24. The Porro system consists of two
right-angle prisms oriented at 90° to each other. The first prism
inverts the image from top to bottom and the second prism reverses it
from left to right. The optical axis is displaced laterally, but is not
deviated. One can see that if this system is inserted into the telescope of
Fig. 7.23, the final image will have the same orientation as the object.
Although the prism system is ordinarily inserted between the objective
and eyepiece (to minimize its size), it will erect the image regardless of
where it is placed in the system.
The Porro prism (first type) owes its popularity to the fact that the
45°–90°–45° prisms are relatively easy and inexpensive to manufac-
ture, with no critical tolerances. However, if the prisms are not
mounted so that their roof edges are exactly at 90° to each other, the
final image will be rotated through twice the angular mounting
error. This is of special importance in binocular systems where the
image presented to one eye must be identical to that presented to
the other.
A shallow ground slot is often cut across the center of the
hypotenuse face of each prism to prevent unwanted grazing angle
reflections from this face which originate from outside the field of view.
See also Fig. 7.39.

Figure 7.23 In a simple tele-
scope, the objective lens forms
a real, inverted internal image
of the object, which is reimaged
by the eyelens. The image seen
by the eye is a virtual inverted
image of the object.