CHAPTER 9 TESTING OPTICAL PROPERTIES OF INSTRUMENTS

TM 9-258

CHAPTER 9

TESTING OPTICAL PROPERTIES OF INSTRUMENTS

known. It can be determined by measuring the focal

9-1. General. This chapter describes a number of

lengths of the elements. The magnifying power can be

relatively simple procedures by means of which the

approximately determined by this method: Mark a

properties and qualities of complete instruments may be

number of connected squares of equal size in a line on a

measured or calculated. For the most part, the tests can

distant white wall. Observe the squares with the right

be made without special equipment. Laboratory test

eye looking through the instrument and with the left eye

methods are purposely avoided.

looking at them naturally. The right eye will receive a

9-2. Objective Aperture. The objective aperture size is

magnified impression of one or two of the squares; the

limited by the clear area provided by the mount of the

left eye will receive a natural impression of a number of

objective. It can be measured in millimeters or decimal

them. The two impressions will be fused by the brain

parts or fractions of an inch with a scale, rule, or with a

into a single impression of a large square with a number

Ramsden dynameter (para 5-24).

of smaller ones crossing it in a line. If four natural

9-3. Measurement of Exit Pupil and Eye Distance.

squares are seen inside the magnified square, the

Direct the instrument at an illuminated area. Focus the

instrument has a magnification of approximately 4

exit pupil (fig 2-54) on translucent paper or ground glass.

diameters or 4 power.

To focus the exit pupil, move the paper or glass towards

9-5. Measurement of Field of View.

or away from the eyepiece until the exit pupil is seen in

a. The field of view (FOV) of an instrument may be

its smallest diameter as a sharp circular shape of even

expressed in either angular or linear measure. The

brightness. Measure the distance of the face of the

angular measure is more commonly given and is

paper or glass from the rear surface of the eyepiece lens

expressed in degrees and minutes. The linear measure

with a scale graduated in millimeters or decimals or

is expressed in meters at ranges of 100 meters or 1,000

fractions of an inch (fig 2-55). Mark the opposite edges

meters.

of the exit pupil on the paper or glass and measure its

b. To determine angular FOV, place the telescope

diameter in millimeters or decimals or fractions of an

in a fixture or mount which is provided with an azimuth

inch. The position and diameter of the exit pupil can be

scale, a micrometer, and levels. Level the instrument

measured exactly by means of a Ramsden dynameter

and direct the telescope so that a convenient vertical

(para 5-24).

object is at the left edge of the FOV. Read the scale and

9-4. Magnifying Power.

micrometer. Traverse the instrument until the object is at

a. Definition. The magnifying power of an optical

the right edge of the FOV. Again read the scale and

instrument is the ratio of the heights of the retinal images

micrometer. The difference between the two readings is

produced in the observer's eye with and without the

the angular measure of the true FOV (para 5-25). If the

instrument.

scale and micrometer are graduated in mils, convert to

b. Calculation. The magnifying power of an optical

degrees and minutes.

instrument depends upon the relation between the focal

c. To measure the linear FOV, place two stakes at

length of the objective and the focal length of the lenses

a specified distance from the instrument; one at each

of the eyepiece, the latter being considered as a single

edge of the observed FOV. Measure the actual distance

lens. The magnifying power of a telescope equals to the

between the two stakes. The linear FOV may be

focal length of the objective divided by the focal length of

determined mathematically by the formula: 2 x tangent

the eyepiece. For example, if the objective has an

of one-half the angular FOV x the range of the stakes.

equivalent focal length of 8 inches and the eyepiece has

9-6. Determination of Spherical Aberration. Cut a

an equivalent focal length of 1 inch, the instrument will

mask of black paper that will cover the objective. Cut a

have a magnification of 8 power.

circle one-half the diameter of the objective out of the

c. Measurement. The magnifying power of an

center of this mask. Apply the mask to the objective.

instrument can be calculated when the equivalent focal

Sharply

focus

distant

object

the

lengths of the objective and the eyepiece are

9-1