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Telescopes
Do the Pre-Lab BEFORE Arriving to the Lab
ALWAYS COME PREPARED
OTHERWISE YOU WILL NOT BE ABLE TO FINISH THE LAB IN TIME!
ALSO THE PRE-LAB IS 20% OF YOUR LAB GRADE.
Picture taken form the Yerkes Observatory site at http://astro.uchicago.edu/yerkes/
This is a picture of the world’s largest refractor telescope, the 40 inch Yerkes Telescope. The
Expansion of the Universe was discovered with this telescope in the 1920’s. The first telescope ever
build was a refractor telescope, one that uses two lenses. (In the large, modern telescopes one of the
lenses is replaced with a concave mirror). Lenses were discovered roughly 700 years ago. They were
mostly used as magnifying glasses, or for visual aids. It took humanity roughly another 300 years to
figure out that if you combine two lenses in a particular fashion, you obtain a telescope. Galileo is often
credited with inventing the telescope in the early 1600s. This is not true. Although he was the first
scientist to point his telescope towards the heavens, he was not the first person to discover the telescope.
An unknown Dutch lens maker invented the telescope. Galileo heard about this new magnifying glass,
and was told that it consisted of two lenses inside a tube. Apparently, he then went home, played with
some lenses and by the next morning had invented the telescope. He called his finding a spyglass and
used this to get a promotion (well, human nature and politics have not changed much since then). In this
lab we’ll do something similar. First we will analyze the properties of lenses. Then, we will somehow
(you’ll figure this out) combine two lenses and rediscover the telescope (or spyglass, if you prefer).
This lab utilizes materials by Project STAR (http://www.starlab.com/psmain.html).
Telescopes  Lab 2  1
Part I — Focal Length
If it is a sunny day, go outside, take a piece of paper, a ruler along with five lenses, and figure out how
to determine the focal length of the lenses. You will have probably done this exercise as a kid… Take
the following five lenses provided by your instructor: a large converging lens, a small thick and a thin
converging lens a small thick and thin diverging lens.
NEVER look at the Sun through a lens!!
[If there is no Sun, darken the classroom and use a DISTANT light bulb. Do your measurements as far
away from the light bulb as possible (at the other end of the classroom), otherwise you will not be
measuring the “image distances” rather than the focal lengths of the lenses.]
a) Converging Lenses
Determine the focal lengths of the converging lenses.
Large Converging Lens
flarge = ________ ± _______ cm
Small Thick Converging Lens
fthick = ________ ± _______ cm
Small Thin Converging Lens
fthin = ________ ± _______ cm
Print the focal length here
Print the margin of error
[Note: “ f = 15 ± 2 cm” means that the focal length is roughly 15 cm and you’d probably believe the
results of other students as long as their numbers are between 13 to 17 cm.]
Explain in words how you determined the focal length of the converging lenses.
Explain in words how you determined the accuracy in the focal lengths.
2  Lab 2  Telescopes
b) Combining Lenses
Compare the (small) thick and thin converging lenses. Which has a larger focal length? ___________
Hold both small converging lenses close to each other (no space):
fjoint = _______ ± _______ cm
Is the focal length of the combined lens longer or shorter than that of the individual lenses? ________
Explain your answer.
Combine the small thick converging lens with the thin diverging lens:
fjoint = _______ ± _______ cm
Is the focal length of the combined lens longer or shorter than that of the individual lenses? ________
Explain your answer. Why do you think this is so.
c) Diverging Lenses
Take the thick diverging lens, and determine its focal length:
fdiverging = _______ ± _______ cm
All lenses have a focal length. Explain how you determined the focal length of the diverging lens. This
is not easy; you will need to do some innovative thinking to figure this out. Consult the Pre-Lab and
compare Figure 4 to Figure 12.
Telescopes  Lab 2  3
Part II — Determining Image Properties
When you look through lenses, you’ll notice that some act as magnifying glasses, while others do not.
Some may even flip the images. For this exercise use only the small lenses.
a) Hold all lenses close to your eye (the way you would wear glasses) and look at an object at the other
end of the classroom. The image will look blurry. Circle the right answer.
Explain in words how the images of converging lens differs from the original
Explain in words how the images of diverging lens differs from the original
Explain how the thickness of the lenses affects the images of converging lenses
Explain how the thickness of the lenses affects the images of diverging lenses
b) Hold all lenses close to your textbook (roughly 1 to 3 inches away from the textbook) and look
through the lens. Circle the right answer.
Explain in words how the images of converging lens differs from the original
Explain in words how the images of diverging lens differs from the original
Explain which lens would you use as a magnifying glass (the thick/thin converging/diverging lens)?
4  Lab 2  Telescopes
c) Hold the lenses at arms length and look at a picture on the wall. Comment on the size and the
orientation of the image as you walk towards the picture on the wall (i.e., as you decrease the
distance between the lens and the picture from roughly 1m; to 50cm; to 10cm; to 5 cm and to 1 cm –
make sure you get very close to the picture on the wall)
Explain what happens to the images of the thin diverging lens as you walk towards the picture.
Explain what happens to the images of the thin converging lens as you walk towards the picture.
At some point the images of converging lenses get very blurry. Find that distance for both lenses:
Thick converging lens:
Distance at which there is no image: __________ ± ________ cm
Thin converging lens:
Distance at which there is no image: __________ ± ________ cm
In part I, you already determined the focal lengths of those lenses. Copy them:
Thick converging lens:
Focal Length:
__________ ± ________ cm
Thin converging lens:
Focal Length:
__________ ± ________ cm
Compare those distances to the focal lengths of the lenses
Explain that answer
Telescopes  Lab 2  5
Part III — Designing a Telescope
Now you know the properties of converging and diverging lenses. The next step is to combine two
lenses such that you get a larger, but clear image. Just image – you get to play Galileo who discovers a
telescope. Go outside the classroom and look at the exit sign at the FAR end of the corridor.
a) Galilean Telescope (aka “the spy-glass”)
First try the combination of the large CONVEX and the small thick CONCAVE lens.
Your task is to arrange the lenses in such a manner that you get a larger, but clear image.
Which of the two lenses would you use as the eyepiece and which as the objective? _____________
EXPLAIN.
Measure the distance between the two lenses:
distance
=
_______ ± _______ cm
Copy the focal lengths from page 2:
flarge-convex =
_______ ± _______ cm
fthick-concave =
_______ ± _______ cm
The distance between the two lenses is larger/smaller than the focal length of the large convex lens.
(Cross out the wrong option.)
Repeat the same using the large CONVEX and the small thin CONCAVE lens.
Measure the distance between the two lenses:
distance
=
_______ ± _______ cm
Copy the focal lengths from page 2:
flarge-convex =
_______ ± _______ cm
fthin-concave =
_______ ± _______ cm
The distance between the two lenses is larger/smaller than the focal length of the large convex lens.
Which telescope magnifies more – the one with a thin or thick concave eyepiece? _____________
Refracting telescopes have problems with chromatic aberration. This is because bluer light gets more
strongly refracted than red light. This effect is most dramatic at the edges of the images, where you
might see rainbow colors. Look at the edges of the images – which configuration has more problems
with chromatic aberration – the one with a thin or thick convex eyepiece?
6  Lab 2  Telescopes
b) Keplerian Telescope
Then try the combination of the large CONVEX and the small thick CONVEX lens. Your
task is to arrange the lenses in such a manner that you get a larger, but clear image.
Did you notice anything strange about the image?
_______________________________________
Measure the distance between the two lenses:
distance
=
_______ ± _______ cm
Copy the focal lengths from page 2:
flarge-convex =
_______ ± _______ cm
fthick-convex =
_______ ± _______ cm
The distance between the two lenses is larger/smaller than the focal length of the large convex lens.
Repeat the same using the large CONVEX and the small thin CONVEX lens.
Measure the distance between the two lenses:
distance
=
_______ ± _______ cm
Copy the focal lengths from page 2:
flarge-convex =
_______ ± _______ cm
fthin-convex
_______ ± _______ cm
=
The distance between the two lenses is larger/smaller than the focal length of the large convex lens.
Which telescope magnifies more – the one with a thin or thick convex eyepiece? _______________
EXPLAIN
Which telescope has more problems with chromatic aberration (thin/thick eyepiece)? ____________
EXPLAIN
Telescopes  Lab 2  7
c) Comparing Galilean and Keplerian Telescopes
Comment on the orientation of the image (KELPERIAN OR GALILEAN)?
How does the magnification of the image depend on the focal length of the eyepiece? (COMMENT
ON THE KELPERIAN AND GALILEAN TELESCOPE)?
How does the field of view of the telescope depend on the properties of the eyepiece? (COMMENT ON
THE KELPERIAN AND GALILEAN TELESCOPE)?
Comment on the clarity of the image of the KELPERIAN AND GALILEAN TELESCOPE? (USE THE THINK
EYEPIECES FOR BOTH TELESCOPES)
Which telescope has more problems with chromatic aberration? You’ll see rainbow colors at the
edges of the lenses. (USE THE THICK EYEPIECE FOR BOTH TELESCOPES)
Suggest an Explanation for your answer.
8  Lab 2  Telescopes
d) Schematic Diagrams of Galilean and Keplerian Telescopes
Consult Figures 4 and 12 of the Pre-Lab. Consult you textbook if you like.
[HINT: The light rays from distant objects are parallel — and the light rays leaving the telescope are
also parallel. (Recall, our eyes can easily focus parallel light from distant objects).] Use a RULER.
Draw the path of three light rays (inside the telescope) as they pass through both lenses.
Label the following: (i) distance between lenses, (ii) flarge-convex, and (iii) fsmall-convex.
Label the focal point of the objective lens in both diagrams.
The Keplerian Telescope
Convex Lens
The Galilean Telescope
Concave Lens
What is the correlation between the focal lengths of the two lenses and the distance between them?
(a) for the Keplerian Telescope, and (b) for the Galilean Telescope?
Telescopes  Lab 2  9
Part IV — Building a Telescope
Now that you’ve played with lenses and the formation of images, you know all you need to know to
design and build your own telescope
First analyze both lenses that are in the telescope kit. One lens is wrapped separately; it is the small
and thick lens. The other is much thinner, but larger, and could easily be mistaken for a piece of
glass – but it is also a lens. Are the lenses converging or diverging? What are their focal lengths?
f large = _________ ± __________
This lens is converging/diverging.
f small = _________ ± __________
This lens is converging/diverging.
Explain how you tested whether the lenses are diverging or converging
Then build the telescope (using Project STAR Telescope Kit: http://www.starlab.com/psprod.html)
Slide the cardboard tube to and fro until you get a large but clear image.
Measure the distance between the two lenses:
d = _________ ± __________ cm
What is this distance in terms of the focal lengths of BOTH lenses?
Does the telescope invert things?
________________
Is this a Keplerian or a Galilean Telescope?
________________
You may keep this telescope,
10  Lab 2  Telescopes
But only if you promise to use it — Otherwise please return it!
Telescopes  Lab 2  11
Lab Report
If you did not do the Pre-Lab before coming to Lab, attach it to this report for partial (50%) credit.
1) Objective of the Lab.
2) Summarize your basic results on the configurations of the Galilean and Keplerian Telescopes.
3) Now you know everything about the orientation, magnification, and sharpness of the image. As an
astronomer, which telescope (Galilean or Keplerian) would you prefer? EXPLAIN.
12  Lab 2  Telescopes
For Extra Credit
4) Reflecting telescopes are somewhat similar to refracting telescopes with the difference that a
concave mirror replaces the objective lens. Read the appropriate paragraphs in your textbook and make
a schematic drawing of a simple reflecting telescope.
5) Today many telescopes are Reflecting Telescopes. What are the advantages of reflecting telescopes
over refracting telescopes?
Telescopes  Lab 2  13