Galilean Telescope.

TM 9-258

unit or by moving only the rear erector element forward,

This negative eyelens is placed so that rays from the

the image formed by the erectors will move back. In this

objective meet it before forming the real image, which

case, it is necessary to move the eyepiece back a

does not exist, and is called the virtual object. Rays

distance equal to the image shift to keep the image in

approaching this virtual object will be bent outward and

focus (B, fig 5-12).

made to diverge as if from the enlarged virtual image (fig

5-13). This renders converging rays from the objective

5-10. Prism Erecting Systems. A prism erecting

parallel before they have converged to form a real

system (para 4-7d) may be used in a terrestrial telescope

image; therefore, no real image exists in this system.

instead of a lens erecting system. In prism, offset, and

The real image, which would be formed by the objective

elbow telescopes, it is placed between the objective and

if the eyelens weren't there, becomes the virtual object

its focal plane. Either all or part of the prism erecting

for the eyepiece and lies at the first principal focus of the

system, however, may be outside (in front of) the

eyelens. In this case, the virtual image viewed through

telescope system. In the BC periscope, the prism in the

the negative eyelens will be at infinity and can be viewed

head is outside the periscope system but still a part of

by the eye without accommodation. This is the zero

the prism erecting system. The prism in the head,

diopter setting (fig 5-14) for this instrument, meaning that

together with the prism system in the elbow below,

all rays from any point source emerge from the eyepiece

comprises a Porro prism erecting system. All observing

parallel. If the eyelens is moved in or out, the emergent

and sighting telescopes used by the Army employ an

rays either will converge or diverge so that the instrument

erecting system.

cap be adjusted for farsighted or nearsighted eyes or for

5-11. Galilean Telescope.

distance. The image viewed through the eyepiece is

a. Basic Principles. The system employed in

erect because the emergent rays are bent further away

the Galilean telescope is based on two major principles.

from the axis instead of recrossing it as in a Keplerian or

(1) It makes use of a negative eyelens

astronomical system employing a positive eyelens.

positioned a distance equal to its focal length (fe) in front

Compare figure 5-13 with figure 5-6.

of the objective focal point (figs 5-13 and 5-14).

5-6