places a second convex of converging lens in the proper
fire-control instruments, a lens erecting system of two
position to pick up the inverted image framed by the
and sometimes three lenses is substituted for the single
objective and forms a second erect image which is
erecting lens (lower, fig 4-20). The entire lens erecting
magnified by the eyepiece (upper, fig 4-20). In most
system functions as a single lens.
Figure 4-20. Lens erecting systems.
(2) In a two-element lens erecting system, both
systems function by internal reflection.
lenses usually are of compound (achromatic) type. In a
exception of rotating prisms, if the path of light is to
three-element system, the third lens is of planoconvex or
continue in an unchanged direction, a total of four
double-convex type and serves as a collective lens. It
reflections is required.
(1) Porro erecting system. This system (fig 4-9) is
converges the light rays of the image formed by the
objective so they will pass-through the erecting lenses. It
employed in prism binoculars and in a number of
increases the field of view and brightness of image for a
telescopes. It tends to decrease the curvature of the
given diameter of the lenses of the erecting' system.
field and its use is particularly advantageous when a
Inasmuch as the collective lens is sometimes placed at
compact instrument having a large field of view is
the point where the image of the objective lens is formed,
desired as is the case in observation telescopes. This
and a reticle must be placed at this same point, the
system is not well suited to telescopes which are to be
reticle markings are simply engraved on the flat face of
mounted gun carriages as the prisms are of awkward
the planoconvex lens used as the collective lens.
shape and it is difficult to clamp them so tightly that they
(3) According to the placement of the erecting
will not shift their positions when the gun is fired. In
lenses, a lens erecting system may increase or decrease
some periscopes and in the battery commander's
the magnifying power of the instrument or it may not
periscope, the 900 prisms in the heads together with the
affect magnification. Moving the erecting lenses closer
lower prisms constitute a Ponrro prism erecting system.
(2) Abbe erecting system.
to the focal point of the objective lens and farther from
the eyepiece increases the magnification but cuts down
system (fig 4-21) employs two double right-angle prisms
the field of view. When the erecting lenses are located
(fig 4-16). The two prisms are assembled in the
at equal distances from the focal points of the objective
instrument to give the same optical effect as in the three
and the eyepiece there is not additional magnification
piece Abbe prism (fig 4-10). The two prisms are not
due to erecting lenses. This method of changing the
joined, a slight amount of space being left between them.
degree of magnification is employed in instruments of
This system, which inverts and reverts, is used in some
telescopes, including those incorporated in periscopes,
d. Prism Erecting Systems.
as an erecting system.