Galileo’s Telescope

Introduction:

Galileo did not invent the refracting telescope. That honor goes to Hans Lippershey of Holland in 1608. His scope was designed for military use. After hearing of an invention that made distant objects appear closer, Galileo built his first telescope in 1609 and was the first person to turn such an instrument to the sky!

The reason that we can’t see objects very well when they are far away is because the light coming from these objects doesn’t take up enough space on our retina (Our light sensor). Many people compare our eye to a digital camera. Distant objects don’t cover enough pixels on our eye’s sensor to be clear. However if we had a larger eye, more light could be gathered, and therefore more “pixels” could be activated to create an image. We could then magnify parts of that image to get a better look. That is how a telescope works! The objective lens gathers light, and refracts (bends) it to create a detailed image at a point called the focus. A larger objective lens gathers more light, and therefore creates a more detailed image. The diameter of the objective lens is referred to as the aperture. The eyepiece then magnifies the image (or portion of the image) that the objective lens created. The distance between the lens, and the image is the focal length. All lenses have a specific focal length.

Pre-Lab:Use your notes and the reading above to answer the following questions before you get started.

  1. What is the function of the objective lens?
  2. What is the function of the eye piece?
  3. Why is magnification not the most important thing?
  4. What is aperture?

In your parts bag find a large lens and a small lens. These scratch very easily so please handle with care.

  1. Does the larger or the smaller lens magnify more? Is this the eyepiece or the objective lens?
  2. Is your answer to the previous question what you would expect? Explain!!!!!!!!!!!!!!!!!!!!!!!!!

Procedure:

Assembly Manual

Imagine that you are Galileo and you have just learned of an instrument called a telescope that may help you study the sky. In front of you are all of the pieces necessary to assemble a refracting telescope. Your first task is to write an assembly manual for how to build a telescope. There are many different paths you can take, but the outcome must be the same; a functioning telescope! Below is a list of the parts to help you get started. A good first step would be to identify each piece. In your assembly manual make sure to refer to each piece by its name, and remember that pictures are very helpful! Each step should have instructions and a picture!

Parts List:

-Large Red Cap

-Small Red Cap

-Objective Lens

-Large cardboard tube

-Small cardboard tube

-Cardboard washer

-Foam eyepiece holder

-Cardboard eyepiece spacer

-Eyepiece lens

Discussion Questions:

  1. Describe the image that you are able to see through your telescope with qualitative observations.
  2. List two challenges to using this telescope, and a practical solution to at least one of them.
  3. What is it called when color doesn’t focus properly when using a refracting telescope? Why does this happen?
  1. Estimating Focal Length:
  2. Move the tubes in and out until the image is as clear as possible.
  3. Use a meter stick to measure the distance (D) between the two lenses in mm.
  4. If the focal length of the eyepiece (Fe) = 25mm, what is the focal length of the objective lens (Fo)?

*Equation: Total Distance (D) = Focal Length of Objective F(e) + Focal Length of Eyepiece F(o)

  1. Percent Error:

It is difficult to measure precisely, and each person may disagree on where the image is focused. It is important to understand the limitations of our procedures.

  1. The actual focal length of the Objective Lens is 400mm. What was the percent error of your measurement?

*Equation: Percent Error = (Actual Value) – (Measured Value) x 100%

(Actual Value)

  1. Adjust your telescope so that the total distance (D) between your two lenses is 425mm does the image look better or worse?
  1. Magnification:
  2. Now that we know the focal length of each lens we can calculate the magnification of our telescope.

*Equation: Magnification = Focal Length of Objective Lens

Focal Length of Eyepiece

  1. What is the magnification of our telescope?
  2. In modern telescopes, how can the magnification be adjusted?
  1. With your supplies, you also should have a card with a smiley face on it. Have somebody in your group hold it upon the other side of the room and use your telescope to look at it. Draw a diagram of your telescope viewing this object. In your diagram label the following: the objective lens, the eyepiece, the focal length of both lenses, and the total distance between the lenses. In your drawing you must show the card, and the image produced at the focus, use arrows to show the light, and what has happened to the image’s orientation. MAKE SURE EVERTYTHING IS LABELED. This drawing should show that you understand how a basic refracting telescope works!
  1. Read the article titled “Galileo’s Telescope Discoveries” and answer the questions below
  1. What are five major discoveries that came from Galileo’s observations?
  2. Based on what you have read, how do you think Galileo’s first telescope compared to the one that you just built?
  3. What importance did each of the groups of discoveries below have to the field of Astronomy:
  4. Craters on the moon and sunspots
  5. Moons orbiting Jupiter, and the phases of Venus
  6. Ptolemy lived from about AD 90 to AD 168 and his model of the solar system was geocentric. Describe his model and explain what geocentric means.
  7. Copernicus lived from 1473 to 1543 and developed a model of the solar system that is heliocentric. Describe his model and explain what heliocentric means.
  8. Whose model did Galileo’s observation support in 1609? Which observations were the most important in supporting one side or the other?