Figure 5-5. Keplerian system

TM 9-258

eye. However, if another positive lens is placed between

such an image and the eye, and the real image lies at

the first focal point of this eyelens, the eye can see

without accommodation of a virtual image of the object

seen by the objective lens. This is the Keplerian system

(fig 5-5). Such an arrangement is a telescope of the

simplest form and is illustrated in figure 5-6 with the

virtual image moved in to the near point of the eye. This

system generally is limited to astronomical observation

becasue the image is inverted. The Army uses this

system only to adjust other types of telescopes and when

it is so used, calls it a collimating telescope.

Figure 5-4. Objective lens.

Figure 5-5. Keplerian system

Figure 5-7. Concave mirror

Figure 5-6. Refracting astronomical telescope

(a) An object beyond center of

curvature forms a real image between center of

5-6.

Reflecting Telescopes.

curvature and focal point. This image is smaller than the

a. Types of Spherical Mirrors.

object and inverted and reverted.

(1) Convex. In this case, the mirror surface is

(b) When the object is between the

on the outside of a spherical surface, the center of

center of curvature and the focal point, the image is

curvature of which lies on the side opposite the incident

formed beyond the center of curvature and is real,

light. This type of spherical mirror produces small virtual

enlarged, and inverted and reverted.

image only. It is frequently used on trucks as a rearview

b. Reflecting Telescope.

This telescope,

mirror.

diagramed in figure 5-8, utilizes a concave mirror of long

(2) Concave. In this type, the light is incident

focal length (instead of an objective lens) to form the real

on the same side as the center of curvature (C) of the

image. This is viewed as a virtual image through an

sphere. The focal point (f) is located halfway between

eyepiece which magnified it or is photographed by a

center of curvature and reflecting surface (fig 5-7). The

camera attachment. The Mt. Palomar telescope, for

concave mirror forms virtual and enlarged images of

example, has a 200-inch diameter mirror of long focal

objects within its focal length which is one-half the radius

length, great light-gathering ability, and great resolving

of curvature. Some of the uses to which it is put are

power. (For resolving power see para 5-30).

headlamp reflectors, searchlight reflectors, and objective

glasses in astronomical telescopes.

5-3