Section III. TERRESTRIAL TELESCOPES

TM 9-258

5-7.

Magnification of Telescopes.

a. Computation of power in an astronomical

telescope is done by dividing the focal length of the

objective by the focal length of the eyepiece. This is true

only when the virtual image is at infinity or when

emergent rays from point object are parallel. If the

image is moved to the near point of the eye (10 inches),

it increases slightly in size.

b. This equation cannot be applied to all

terrestrial systems using lens erecting systems since

such erecting systems can, and usually do, contribute to

the power. It can be applied to any terrestrial system

using a prism erecting system.

Figure 5-8. Reflecting astronomical telescope.

Section III. TERRESTRIAL TELESCOPES

the eyepiece to erect the image as in figure 5-9. A prism

5-8.

General. This instrument gets its name from the

erecting system is placed between the objective and its

Latin term "terra" which means earth and is basically

focal point, but a lens erecting system requires

useful for looking at objects as they actually appear to us

repositioning of objective and eyepiece so that erectors

on earth. Any astronomical telescope can be converted

are between objective focal point and first principal focus

into a terrestrial telescope by inserting an erecting

of eyepiece.

system. either lens or prism, between the objective and

Figure 5-9. Terrestrial telescope--simple form.

erecting system. The first lens may be placed one focal

5-9.

Lens Erecting Systems.

length behind the image formed by the objective and the

a. Location. A lens erecting system is placed

second spaced a distance equal to the focal length of

between the focal plane of the objective and the front

either to minimize spherical aberrations (fig 5-10).

focal plane of the eyepiece. In practice, to minimize

aberrations, two or more lenses usually comprises a lens

5-4