Optical Powers
Size in the Sky

Class Date

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Activity 3a - Size in the Sky

  1. List at least five facts about Jupiter and our moon. If you can, place the facts in pairs that compare a characteristic such as composition or rotation.


The Moon
















  1. Open your HOU-IP software.  Locate folder 3a-SizeSky.  Open the images: 3a1-moon.fts and 3a1-jupiter.fts. Then tile the images.  Go to Window, Tile on the menu bar.

    Open 3a-SizeSky images, 3a1-jupiter.fts and 3a1-moon.fts. 

Both these images were taken with the same telescope CCD camera. The large crater in the middle of the moon image is called Copernicus. With the Jupiter image selected, click on “log scale” on the tool bar. Look for the four Galilean moons.

  1. Describe some of the features you see in each image.


  1. Subtract the images to compare the size of Jupiter to the size of the crater, Copernicus.
    1. Click on the moon image to select it as the active window. 
    2. On the Menu bar, go to Manipulation.
    3. Click Subtract.
    4. Select “displayed image.” In the box that appears to the right of this selection, choose the Jupiter image.
    5. Be sure to check the “Display results in new window” box at the bottom of the window.
    6. Click O.K.
HOU Menu - Manipulation, Subtract Select Moon Image, Subtract Jupiter Image.


  1. What happened?  Describe what you see in the new image.



  1. The instructions indicate that the two images were taken with the same telescope and CCD camera. Why is this important?  

    Close the images.  When asked: "Save changes?", choose No.

  1. Now open images, 3a2-copernicus.fts and 3a2-jupiter.fts. Tile the two images.
  1. Select the Copernicus image and add the Jupiter image to it.  (See instructions in #4 above.) In this step you will add instead of subtracting the images. These two images were taken with the CCD camera on a telescope with greater magnification.   Even with the greater magnification, how does the apparent size of Jupiter compare with the apparent size of the crater, Copernicus? 



  1. What conclusions can you draw from what you have seen in this investigation?



  1.  Define angular size.



A Bigger Challenge –

  1. Open images 3a2-copernicus.fts, 3a2-jupiter.fts, 3a2-saturn.fts, and 3a2-ringnebula.fts.  Now open 3a2-combo.fts.  Try different image processing tools to help you uncover the components of this combo image.
    1. Change min/max
    2. Log Scaling
    3. Choose a different color palette from the tool bar.
  2. Propose an explanation of how this image was made.



  1. What have you learned about the Ring Nebula?



  1. Make your own new combo image and challenge your friends.
  2. All these images were taken with the same telescope and CCD camera so their apparent sizes are comparable.  The moon crater, Jupiter and Saturn are all in our solar system.  The image of the Ring Nebula shows an object very distant from the solar system, yet still inside our own Milky Way Galaxy.  If the Ring Nebula is very distant from us (2,300 light years), yet appears larger than Jupiter and Saturn, what does that mean about the real size of the Ring Nebula compared to Jupiter and Saturn?

Another Challenge - Size in a Revolving Solar System

Something else to think about is how the apparent size of solar system objects might change depending where Earth is with respect to the other  planets.  The Earth and other planets revolve around the Sun.  As we change position throughout the year and as the other planets also change positions, apparent sizes also change.  

Take a look at two pictures of Jupiter taken a two and a half months apart.  You can find the exact dates under Image Info.  The distance between Earth and Jupiter changed from 4.669 astronomical units (au) to 4.064 au.  (An astronomical unit is the average distance between the Earth and the Sun.)  Compare the apparent sizes of Jupiter on these two dates.  Measure the sizes using the slice tool.  Compare the ratios of the slices to the ratios of the distances. 

Summarize your findings. Interpret your results.