High-Powered Instruments

TM 9-258

eyepiece to a single focus setting at -0.75 to -1.0 diopter.

available for checking zero diopter setting.

A single focus setting will be satisfactory to any observer

whose eye correction is not too great. In a low-power

instrument, the objective is short focal length; therefore,

the image plane shift is small when target distance

changes unless the target is moved very close.

e. High-Powered Instruments. In high-power

instruments (more than 4x) the diopter movement is

useful because the eye relief frequently is too short

(excepting in rifle scopes and gunsights) to permit view

of full field while wearing spectacles. If the user's

defective sight is not caused by astigmatism (is either

farsighted or nearsighted) and the correction, required is

within the range of the 'diopter movement, the diopter

adjustment permits adjustment of the eyepeice to correct

for his defective vision. This eliminates the necessity for

Figure 5-22. Dioptometer

spectacles while using the instrument. In a high-power

instrument not having a reticle, the diopter movement

5-16.

Collimation.

permits focusing the eyepiece on the different image

a. Definition. Collimation is the alinement of

planes, resulting when the instrument is used at all

ranges, thus focusing the instrument for the different

the optical and mechanical axes of an instrument and

ranges. An example of this is the observation telescope

necessitates a reticle in the collimating telescope so it

which can be used from approximately 50 feet to infinity.

can be used as a telescopic sight. (for reticles, see para

Diopter Scale. The diopter scale on an

5-19). Alinement can be obtained by sighting through a

eyepiece is calibrated in units indicating the movement of

fixed collimating telescope and the instrument to be

the eyepiece necessary to effect a change of 1 diopter in

collimated, and adjusting accordingly.

b. Lens Decentration. Lens decentration is

the emergent light. For example, at plus 1 the angular

convergence is the same as with a 1-power lens (focal

utilized frequently in collimating optical systems,

length of 1 meter) and the rays will focus or cross at 1

especially binocular instruments, which usually employ

meter behind the eyepiece. At minus 1 the angular

objectives mounted in eccentric rings (para 4-18) to

divergence is such that the rays, if extended back into

permit movement in any direction across the axis. If a

the instrument, would cross at 1 meter forward of the

lens is decentered (moved across the axis of the system)

eyepiece.

it will cause an image shift in the same direction. The

optical center seldom coincides with the exact

5-15. Dioptometer. The dioptometer (fig 5-22) is a

geometrical center of a lens so that an image shift

precision instrument useful in determining accurately the

frequently will result from revolving a lens in its cell. In

diopter setting of another instrument. It is placed

fact, disturbing any lens in a collimated system will

between the eye and the instrument to be checked. The

necessitate collimation of the system again as any lens

focusing sleeve of the dioptometer is then adjusted until

which fits freely enough to facilitate removal will almost

the field is in sharp focus. The diopter setting of the

surely not go back in its exact position.

instrument being checked is then read from the focusing

c. Prismatic. If prisms are employed in a

sleeve (usually calibrated in 0.1 diopter) of the

system, collimation also can be affected either by sliding

dioptometer. Set at zero diopter the dioptometer can be

prism parallel to the plane of reflection (causing an

used to check-the accuracy of the zero diopter setting in

image shift in same direction) or by tipping prism and

the collimating telescope itself which is commonly

plane of reflection (shifting image in same direction).

5-11