Section VI. FUNCTIONS OF TELESCOPIC SIGHTS

TM 9-258

Section VI. FUNCTIONS OF TELESCOPIC SIGHTS

designed to measure angular distance between two

5-17. Definition.

Terrestrial telescopes having a

points (stadia lines in a transit or grid lines in a military

reticle in a focal plane in the optical system are

sight). As it is in the same focal plane as a real image, it

commonly known as telescopic sights or military

appears superimposed on the target and can be viewed

telescopes.

Such a reticle can be placed in an

by the eye with the same accommodation required for

astronomical telescope, as in the collimating telescope,

viewing the target or field.

but a telescopic sight is universally considered to be an

b. Location. In an optical system employing a

instrument having an erecting system.

lens erecting system, there are two possible reticle

5-18. Advantages.

telescopic

sight

locations (fig 5-23). It may be placed either in the image

advantageous in that it permits the viewing of reticle and

plane of the objective or at the focal point of the eyepiece

target in the same optical plane and does not require

(image plane of errectors). If the erecting system

precise alinement of the eye with respect to the line of

increases the magnification and the reticle is in the

sight. Alinement of the eye need be only within the exit

image plane of the objective, the reticle lines will appear

pupil diameter. In comparison, open rifle sights require

wider on the target than if placed at the focal point of the

the eye to attempt an actual impossibility; the focusing

eyepiece. For this reason, the reticle in a low-power rifle

simultaneously on rear sight, front sight, and target. The

scope usually is placed in front of the erectors and in

eye, furthermore, must be in perfect alinement with all

high-power target-type rifle scopes, it is at the focal point

three.

of the eyepiece. With a lens erecting system, the reticle

5-19. Functions of Reticles.

may be placed at the front or rear of the erecting system,

a. Application. Reticles (para 4-9) are used in

or reticles may be placed at both points, and their

fire-control instruments for superimposing markings or

patterns are then observed superimposed upon each

a predetermined pattern of range and deflection

other. The preferable position with this type of erecting

graduations on a target. A reticle in its simplest form is a

system is in front of the system as the objective and

post or picket, or it may consist of two intersecting lines;

reticle then form a unit and any shift of the erecting

then the line of sight through their intersection will be in

system does not disturb the alinement of these elements.

the center of the field of view. It represents the axis of

With a prism erecting system, the reticle usually is

the bore of the weapon when adjusted for short range

placed in back of the erecting system.

firing or is fixed at a definite angle to the bore of the

weapon for long range firing. A reticle is used as a

reference point for sighting or aiming, or it may be

Figure 5-23. Reticle location

of the principle of universal focus through a small

5-20. Galilean-Type Rifle Sight.

In this British-

aperture (familiar to most marksmen). In such a system,

devised system, a long focal length objective (bearing a

it is impossible to have both reticle and target image in

reticle to its front surface) is mounted in place of an open

sharp focus simultaneously. Note also that the reticle

front sight. A negative eyelens and aperture replace the

actually is outside the system. A reticle cannot be placed

conventional peepsight. The aperture provides sharper

within this system as no real image plane

definition

and

limited

parallax

use

5-12