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TM 9-258
both eyes of an individual, provision must be made to
movement. The two lenses of the eyepiece are mounted
adjust the spacing between the eyepieces of the
in a simple tube and its distance from the reticle or focal
instrument to conform to the interpupillary distance of
point of the objective or erecting system can be adjusted
different observers. The interpupillary distance of such
by rack and pinion, by a simple draw tube or by rotating
instruments generally is adjustable from 58 to 72
the entire eyepiece causing it to screw in or out.
d. On fire-control instruments a graduated scale
millimeters because the eye spacing of nearly everyone
is within this range. If the interpupillary distance of an
generally is provided around the eyepiece. This scale is
The diopter is the unit of
individual is greater than 72 millimeters or less than 58
calibrated in diopters.
millimeters, he is incapable of using instruments of this
measurement of the converging power of lenses (para
type to the best advantage.
10-2). For a normal eye the scale on the eyepiece is set
at zero.
If a positive 2-diopter spectacle lens is
commonly worn, the eyepiece should be set at +2
2-36.
Focusing.
provided the spectacles are removed as they should be.
After carefully focusing the instrument, the reading of the
a. For perfect focusing of a monocular instrument
eyepiece should be memorized and used for future
under all conditions, two adjustments are necessary. If
focusing.
the instrument has a reticle, means must be provided for
e. Low-power telescopes are frequently made
adjusting the distance between the reticle and the
without any means for focusing. Such an instrument is
objective so that there will be no parallax (para 2-46).
termed a fixed-focus telescope. When assembled, such
Also, the distance between the eyepiece and the reticle
an instrument is often so adjusted that light from a
must be adjusted for the eye of the observer.  In a
distant source emerging from the instrument, instead of
binocular instrument, it also is necessary to adjust the
being essentially parallel, appears to diverge from a point
instrument to conform to the interpupillary distance of the
40 to 80 inches in front of the eyelens. A telescope so
eyes of the observer.
focused (approximately minus 3/4 to minus 1 diopter) is
b. If the focal length of the objective is short and
more readily adaptable to the eye of the average
the targets in general are at a great distance, the
observer than one focused so that the rays of light are
objective may be adjusted for a target at an infinite
essentially parallel. Because a fixed-focus instrument is
distance when the instrument is assembled in the factory
simple, it can be made entirely waterproof. However, if
and no field adjustment is provided. This is the case with
the power of the telescope is greater than 3. 5 or 4, the
some fire-control instruments and low-power telescopes.
accommodation of the average eye is not sufficient to
c.  Focusing eyepieces are commonly found on fire-
permit its use.
control instruments. This adjustment is primarily
designed for focusing the instrument for different eyes
and is referred to as the diopter
Section VII. ABERRATIONS AND OTHER OPTICAL DEFECTS
coma, curvature of image, and distortion. Other factors
2-37.
General.
which may affect the operation of the optical system of
an instrument are resolving power, Newton's rings, light
a. Aberration is a lens or prism imperfection
loss, and parallax.
resulting in an image that is not a true reproduction of the
object.
2-38.
Spherical Aberration.
b. In designing an instrument, correction is usually
made for optical defects with special attention being
a. Light rays refracted through a lens with spherical
given to the use to which the instrument is to be put.
surfaces near its center and those refracted through the
Correction is achieved by using lenses or prisms made
outer portion or margin do not intersect the axis at a
of two or more kinds of glass (called compound lenses or
single point.  The outer rays of a convergent lens
compound prisms), and by eliminating rays which would
intersect the axis closer to the lens than the more central
be refracted through the outer edges of lenses (called
ones (fig 2-56) and the opposite is true of a convergent
marginal  rays)  by equipping  the  instrument  with
lens, considering the imaginary extension of the
diaphragms (field stops) (para 2-38 e), and a suitable
refracted rays (fig 2-57). The result is a blurred image.
eyepiece.
This fault is common to all single lenses with spherical
c.  There are six general types of aberrations:
surfaces and is termed spherical aberration.
spherical and chromatic aberrations, astigmatism,
2-42

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