Figure 4-38. Diopter scale.

TM 9-258

4-22. Optical Bars.

a. In a range finder, the angles of the lines of sight

from both ends of the instrument are employed to

determine the range. Heat and cold cause unequal

expansion and contraction of metal parts disturbing the

angular alinement of elements mounted on or in them.

The angles determined by the range finders are so small

that an excessive error could be introduced by the

deviation caused by any misalignment if special

measures were not taken to keep the optical systems

stable and in perfect alinement.

b. One of these measures consist of mounting the

most sensitive parts of the instrument in a metal tube

known as the optical bar. The optical bar is made of

specially composed metal with a low coefficient of

expansion. It is perfectly balanced and is supported by

the inner or main tube of the instrument in such a

manner that the outer tube may expand or contract

without affecting the alinement of the inner tube. In

addition, the instrument is insulated to reduce

temperature changes to a minimum.

Figure 4-38. Diopter scale.

4-37