Figure 5-25. Exit pupil-virtual image of objective.

TM 9-258

b. Plane.

In figure 5-25, rays 1 and 2

originating at opposite limits of the true field (para 5-25)

cross at a common point in the objective lens and

recross back of the eyepiece in the plane of the exit

pupil. In the exit pupil plane must be the point of rotation

of the observer's eye if the observer is to see the full

field.

c. Diameter. The diameter of the exit pupil can

be measured by pointing the telescope toward a light

source (out a window) and inserting a piece of translucent

material in the plane of the exit pupil (para 9-3). The

Figure 5-26. Ramsden dynameter.

diameter of the image then can be measured on the paper.

This diameter also can be approximated by holding the

b. Purpose of Measurement. Measurement of

telescope away from the eye and measuring across the

exit pupil and entrance pupil diameters of an instrument

eyelens the diameter of the bright disk seen through the

is useful in determining the power of an unknown, or for

eyelens or it can be measured quite accurately by using a

example, a foreign instrument by the use of the equation:

dynameter (para 5-24).

Power = AP; (para 5-21).

5-24.

Ramsden Dynameter.

a. Basic Use. The Ramsden dynameter

If the power is known and the diameter of the exit pupil is

(fig 5-26) is used in measuring the position and diameter of

desired (and a dynameter is unavailable), it is usually

the exit pupil. The dynameter is essentially a magnifier or

easier to measure approximately the entrance pupil and

eyepiece with fixed reticle. The eyepiece and the reticle

compute the diameter of the exit pupil.

move as a unit within the dynameter tube. When in use,

5-25. True and Apparent Fields of Telescope.

the dynameter is placed between the eye and the eyepiece

a. Definitions. The true field of view in a

of the instrument and focused until the bright disk of the exit

telescope is the width of the target area or field that can

pupil is sharply defined on the dynameter reticle. The

be viewed. It is expressed as either angular true field or

diameter of the exit pupil is then measured directly on the

linear true field (figs 5-27 and 5-28). The former is the

dynameter reticle (usually graduated in 0.5 millimeters) and

angle at the objective included by the two extreme

the eye distance or eye relief is read from the scale on the

principal rays which will enter the observer's eye. The

dynameter tube.

5-14