Optical Powers
Image Quality

Teacher Page
Activity 2 Image Quality

Standards (see Appendix A):

Unifying Concepts: Form and Function; Systems.


Students become familiar with image processing software and display tools.  Students qualitatively describe differences in images, and begin to use vocabulary such as brightness, magnification, resolution and field of view to describe astronomical images.


Students explore a variety of images, determine in what ways they are different from one another and consider qualities that make images poor or useful.  Students look at selected images of Jupiter, Saturn, nebulae, star clusters, and the Moon taken with different telescopes, or different CCDs, under different conditions.  They use these observations to identify differences such as field of view, brightness, focus, magnification, saturation, exposure, image size, and resolution. Some images are purposefully of poor quality to make it easier to identify these factors.  Once students have identified a concept such as “you can see more of the moon in this image,” you can introduce the correct vocabulary, “the field of view is bigger.”   Encourage students to hypothesize reasons for image differences.




Be sure the following images are accessible from the computer lab.  The following image descriptions are given for the benefit of the teacher. They highlight some but not all of the qualitative differences of the image sets.


Hercules Cluster These images emphasize field of view and image size and scale. Pictured here are clips from two of the images, a and d.

  • 2-hercules_cluster-a.fts Only center of cluster because of small field of view.
  • 2-hercules_cluster-b.fts Larger field of view shows more of the cluster.
  • 2-hercules_cluster-c.fts Even larger field shows more of cluster.
  • 2-hercules_cluster-d.fts Very large field of view compared to previous images. The cluster appears very small. V filter.
  • 2-hercules_cluster-e.fts Same size and scale as image-d.  More and brighter stars but more difficult to see individual stars at center of cluster. No filter.


Jupiter Click on log / log off to see the moons or to see detail on planet.

Lagoon Nebula Compare Field of View in two images. Consider changing the zoom factor, by replacing the 1 with a 2 or a 3 to compare image a to image b.


Orion Nebula This set of images varies greatly in image size and field of view. Some of the images are saturated, with signal overflowing to adjacent pixels in a phenomena called blooming.

Saturn  Three clips of the Saturn images are pictured here, a, b, and c. Some of the images of Saturn also include the moons. Titan is the brightest moon, the others are dimmer and harder to find. If students click on Log on the toolbar of HOU-IP, it will be easier to find the moons.

  • 2-saturn-a.fts  Notice rings of Saturn and moons of Saturn.

  • 2-saturn-b.fts  Compare image size to previous image. The magnification is much less on this image.

  • 2-saturn-c.fts  Image is out of focus.

  • 2-saturn-d.fts  Image size is very small; resulting image of Saturn is tiny.

  • 2-saturn-e.fts  Image is over-exposed, some pixels are saturated, and overflow to others in phenomena called blooming. Notice starry background. Imaging stars and a planet as bright as Saturn is next to impossible without over-exposing Saturn.

  • 2-saturn-f.fts  Image of Saturn is on an angle because camera is on an angle. Image is not oriented north/south. Focus is pretty good but not as good as saturn-a.fts.


  approximately 45-60 min.


  1. Show students how to open the HOU image processing software and where to find the image sets. Divide the class into convenient sized groups.  Assign each group an image set that they will be responsible for.  There is an optional Student Page available for this activity.  The Tips 'n Tricks guide is a helpful reference especially if students are not familiar with HOU image processing software.
  2. Introduce the task to your students: Each set of images is of the same target.  Each image was taken using a telescope and a CCD camera.  A CCD is very similar to a digital camera.  The image is recorded on a computer chip instead of film. Some of the images are of good quality and some are not. They differ in other ways as well.  In order to begin understanding astronomical images like these, we are going to start by looking at how these images are different from one another.
  3. Instruct each group to examine their images carefully, describe how they differ and select one person to report their findings to the class.  Encourage groups which finish quickly to examine additional sets.
  4. As each group reports to the class, write down the main points on the chalkboard or overhead.  Put a checkmark next to a point each time it is repeated.
  5. When the groups have finished reporting their conclusions write the following vocabulary words on the board – Focus, Field of View, Brightness, Magnification, Image Size, Image Scale, Resolution.  Write out the definitions of any words the group does not understand. Explain that these are words that astronomers use to describe images.  As a class, come to consensus about which astronomy words could replace their descriptions.


After you complete the discussion of the vocabulary in step 5 above, have students return to their groups and assign vocabulary to their observations.  Keep this work to refine understandings as the class builds understanding of the concepts during the following activities.