Figure 4-20. Lens erecting systems.

TM 9-258

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.

With the

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

This erecting

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

variable power.

telescopes, including those incorporated in periscopes,

d. Prism Erecting Systems.

Prism erecting

as an erecting system.

4-22