CONSTRUCTION OF A TELESCOPE by Dr. Jim Roberts,
Professor of Physics and Material Science
CONSTRUCTION OF A TELESCOPE
OBJECTIVE: This exercise is designed to acquaint the student with some of the properties of lenses, to find the
focal lengths of two lenses and make these lenses into a telescope. They will learn how to calculate the
magnification of the device and how to estimate the apparent sizes of images over the object size.
INTRODUCTION
The earliest knowledge of a device that served as a telescope can be attributed to a Hollander, Hans
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Lippershey, who was a spectacle manufacturer. He noted that the combination of two lenses gave a gain in image
size for objects being viewed by the combination of the two lenses. This prompted Galileo Galilei to make a device
and use it for practical purposes, such as making astronomical observations and to use it in searching the horizon for
incoming ships far at sea. Although Galileo is given a lot of credit for the invention, Lippershey was successful in
marketing the items as "Dutch Trunks" which were sold to the Dutch army for observation. In only a short time the
telescope (tele-far and scope to see) became objects of scientific and domestic value. Isaac Newton quickly
understood the optics of the device and manufactured his own reflecting telescope which today is referred to as a
Newtonian device. Once the method was understood, devices in various forms appeared in usage.
Two types of telescopes came into being, the refracting telescope and the reflecting telescope. The optics of
each is interesting. We will make a simple refractor in this exercise and study its properties. If time permits, we will
make a reflecting telescope out of make-up mirrors to demonstrate the nature of each type, the refractor and the
reflector telescopes.
PART I
DESIGN AND CONSTRUCTION OF A REFRACTOR TELESCOPE
In figure 1 can be seen the basic design of a refractor telescope. The object consists of only two lenses, one the
objective and the other the eyepiece lens. Each has distinguishing focusing properties. One of the goals of this
exercise is to determine the focusing properties, i. e., the focal lengths of each.
The focal lengths are easily determined by using the sun to establish where the light is brought to focus by the
device. The lens is place in the beam of sunlight or a bright light source and the beam is concentrated by the lens into
its most intense point, as shown in figure 1A. Each of the lenses is tested this way and the length from the center of the
lens to the bright spot measured. These distances become the focal lengths of each instrument. Record these
numbers in the table provided for future reference.
The points P1 and P2 are obtained as the brightest illuminations of the light source on a piece of paper.
Select two paper cylinders that will hold the lenses provided and cut them to suitable lengths. These lengths
are determined by knowing the focal lengths of the two lenses. The total length of the tubes should be about 3 to 6
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inches longer than the total length of the two focal lengths.
The paper cylinders can be collected from several sources, the center roll of paper towels, the center roll of
toilet paper, shipping cylinders for maps, etc. The center roll of paper towels and toilet paper rolls fit nicely into each
other. The combination is too short for a telescope of magnification more than about 3X.
Once the focal lengths of the lenses have been determined, the device can be assembled for testing.
TESTING THE INSTRUMENT
The magnification of the telescope can be determined by sighting through it at an object and estimating the gain
is size when the object is viewed with the telescope and when it is viewed with the naked eye. This estimate can be
further confirmed mathematically by taking the ratio of the length of the objective focal length to the eye piece focal
length.
Figure 1. A sketch of a basic lens combination for a telescope and two lenses with light projection to determine their
focal lengths.
The focal length is expressed as:
Magnification= (Focal length of the objective lens)/(focal length of the eyepiece lens)
These two answers should agree, but the mathematical one is the more accurate.
Find two cylinders that slide in and out of each other as shown in the right side of figure 1. The total length
should be greater than the sum of the two focal lengths Fo and Fe so you can adjust the focus of the image. Tape the
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lenses in the ends of the tubes as shown. The one with the longer focal length is the objective and faces the thing to be
observed. The shorter focal length is the eyepiece and is placed in the end that you look through.
Look at different objects around the room and outside the window and focus the apparatus by adjusting the
tubes holding the lenses. This will enable you to experiment and understand better the instrument.
The device can
be treated as a toy and you should learn everything that you can about it.
QUESTIONS
When you have completed the construction, answer the following questions about the telescope.
1. Is the image larger or smaller when you look through either end of the telescope?
2. How much larger or smaller is the object when viewed through the instrument?
3. Is the image upright or inverted when viewed through the instrument?
4. List some uses that the instrument may have.
5. How can you improve the instrument?
6. Take the instrument outside at night and look at some objects in space, such as stars, planets and the moon and
describe what you see.
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