Section VII. OPTICAL FACTORS IN TELESCOPE DESIGN

TM 9-258

exists. Also, the negative eyelens alone would minify the

simultaneously with the same accommodation. Although

reticle and require eye accommodation to focus on it so

this system is of basic optical interest, it has no practical

that it would be impossible to see both field and reticle

military application in this country.

Section VII. OPTICAL FACTORS IN TELESCOPE DESIGN

This formula is applicable to all types of telescopes.

5-21. Magnification or Power of Telescope. Power

in a telescope can be determined by dividing the

NOTE

diameter of the entrance pupil by the diameter of the exit

Power in optical instruments is denoted

pupil.

by the letter x. For example, a 6 x 30

Power = AP

binocular is 6-power and has an

where AP is the diameter or aperture of the entrance

entrance pupil or objective size of 30

pupil and EP is the diameter of the exit pupil (fig 5-24).

millimeters.

Figure 5-24. Entrance and exit pupils.

5-22.

Entrance Pupil.

5-23.

Exit Pupil.

a. Function. The entrance pupil diameter is

a. Location. The diameter of the bundle of

limited by the clear aperture diameter of the objective

light (figs 5-24 and 5-25) leaving an optical system is

determined by the size of the exit pupil. If an instrument

lens. This clear aperture is limited by the inside diameter

is held at arm's length, the exit pupil is seen as a virtual

of either the lens cell or the retaining ring as indicated in

image of the aperture stop and will appear as a bright

figure 5-24. The entrance pupil can be viewed as such

disk of light (fig 5-25). The position of the exit pupil can

from the objective end of the instrument.

b. Measurement. The entrance pupil can be

be determined by directing the telescope towards an

illuminated area, such as the sky, and by holding a piece

approximated by measuring directly across the objective

of translucent paper or ground glass behind the eyepiece

with a transparent metric scale. Usually the above

in the place where the emergent beam is smallest and

method is sufficiently accurate for most practical

most clearly defined. This disk of light formed at the exit

purposes.

pupil is called the eye circle or Ramsden circle.

5-13