CHAPTER 5 ELEMENTARY PRINCIPLES OF TELESCOPES

TM 9-258

CHAPTER 5

ELEMENTARY PRINCIPLES OF TELESCOPES

Section I. INTRODUCTION

the eye. It should be remembered that rays from a

5-1. History of Optical Instruments. The earliest

distant point are, for all practical purposes, parallel and

record of the use of a lens as a magnifier is estimated to

that the eye can focus or form an image with these

have been in the eleventh century and attributed to an

parallel rays without accommodations. If the object is at

Arabian philosopher names Alhazen. In the thirteenth

the focus (focal point) of the lens, it is seen by the eye

century, spectacles were devised based on work done

without accommodation as the emergent rays are then

both by Roger Bacon and by a Florentine monk named di

parallel (as if originating at a distant object) as illustrated

Spina. But as the seventeenth century opened, nobody

in figure 5-1. If the object is within the focal length of the

had as yet devised a telescope. Quite by accident a

lens, accommodation by the eye is necessary as the

Dutch optician, Jan Lippershey, made the discovery in

emergent rays no longer are parallel but are diverging,

1608. His telescope used a 1 1/2-inch diameter positive

as if from a near object, as in figure 5-2.

(convergent) objective lens of 18-inch focal length and a

negative (divergent) eyelens of 2-inch focal length.

Lippershey was refused a patent by the Dutch

government because of confusing counterclaims from

supposed competitors but in the spring of 1609 the

Italian scientist Galileo carried on the work.

duplicated the original using an instrument of 8 power.

Not satisfied, he made another instrument 37 inches long

of 8 power and still another 49 inches long with a very

short focus eyepiece giving 33 power. His famous

astronomical discoveries were made with the latter

instrument and the optical system he used is still known

as the Galilean telescope. A refracting astronomical

telescope of modern design, using a positive objective

Figure 5-1. Simple magnified-object at principal focus

lens, and a positive eyelens, was designed by the

German astronomer Kepler in 1610 and some 30 years

later a Scottish mathematician named Gregory designed

the first reflecting telescope which is the basis of many

modern astronomical instruments. In the latter part of

the nineteenth century, buffalo hunters on our western

plains used telescopic sights on their rifles.

The

telescopic sight, the most accurate of all sighting

devices, is too easily damaged to be an item of issue

with the infantry rifle except those weapons used by

snipers. It is used, however, on all types of field pieces

and Naval rifles. The remainder of this text is devoted

mainly to a study of the principles involved in such

applications of telescope as telescopic sights and

observation instruments.

5-2. Simple Magnifier. The simple magnifier consists

Figure 5-2. Simple magnifier-object within focal length

of a positive (converging) lens at the first focal plane of

the eye, although this positioning is not too important. If

5-3. Compound Microscope. The real purpose of the

the object being viewed is at or within the focal point of

microscope is to aid the human eye, essentially a long-

the lens, a virtual, erect, and enlarged image is seen by

range high-acuity instrument, to be useful at short dis-

5-1