Figure 8-24. Fundamental triangle of range finder.

TM 9-258

b. The purpose of a range finder is to find the

angle is variable, depending upon the distance of the

target from the instrument. The angle to which the 90ƒ

range of an object or target. This instrument measures

distance by triangulation.

and the variable angle converge is termed the parallactic

c. The range finder contains two optical

angle. It becomes larger as the distance to the observed

systems which permit a single observer to view the target

object becomes less. Because of the extreme shortness

from points some distance apart. This distance serves

of the base line in comparison with the range to be

as the known leg of the triangle. It is termed the base

computed, the utmost accuracy and precision are

line (fig 8-24) and is one form of designation of a range

required in the determination of the angles at the two

finder, that is, a 1-meter base instrument. One of the

ends of the base line.

two angles from which the target is observed is fixed at

900;

the

other

Figure 8-24. Fundamental triangle of range finder.

d. In the stereoscopic range finder, by turning

method could not be used to advantage with a very

rapidly moving target.

the range knob, the target is made to appear in the same

distance plane as the central measuring mark (pip) of the

stereo reticle, when "stereoscopic contact" is

8-22. Range Finder, Tank (Typical Coincidence

established.

Type).

e. In the coincidence range finder, the two

optical systems are focused into a single eyepiece

a. General. The range finder illustrated in

assembly. When the range knob is rotated, the left and

figure 8-25 is typical of the coincidence-type instruments

right fields of -view are superimposed and "coincidence"

designed for use as the principal ranging device in the

of the two images of the selected target is established.

primary sighting system of tanks, and is used for direct

f. A very simple and practical type of

fire operation. This range finder is designed to provide

stereoscopic range finder, based on still another

automatic and continuous ranging information through an

principle, consists of a binocular telescope having a

output shaft to the ballistic computer. For sighting

scale in each ocular. The two scales will fuse into a

systems that do not include a ballistic computer, range

single scale, in relief, having the appearance -f a row of

information may be read from the range scale. Accuracy

dots receding to infinity. These dots are seen alongside

of the range finder is inherently high because of its long

the scene observed and every object in the scene

base length, 79 in., and 10-power magnification. The

appears to be located in the same plane as one of the

range finder may also be used, if the gunner's periscope

dots.

Distance to rapidly moving objects can be

is out of commission, for boresighting the gun. Either the

determined approximately very quickly by this method,

left or right end of the range finder may be used for

while a slower

boresighting.

8-21