### Teacher Page Activity 2 Image Quality

Standards (see Appendix A):

• Science Content Standards: A. Science as Inquiry, D. Earth and Space, E. Science and Technology.
• Mathematics Standards: Representation.
• Technology Tools: Productivity; Research; Problem Solving and Decision Making.

Unifying Concepts: Form and Function; Systems.

Objective:

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.

Overview:

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.

Background:

Vocabulary

• Angular size - the apparent size of an object as measured by the angle between two lines of site along each side of the object.  On images, the angular size is a function of the telescope optics and the CCD imager’s pixel dimensions.
• Brightness - Counts is a measure of brightness that is related to the number of photons collected by picture elements (pixels) in a CCD camera.
• Blooming - when the signal from saturated pixels flows into adjacent pixels.
• CCD - Charge Coupled Device, a camera that uses a light-collecting chip to digitally record how much light (photons) strike a specific place (picture element-pixel) on the chip. This camera attaches onto a telescope in place of the eyepiece.  See the HOU On-Line course for graphical explanation.
• Exposure time - the amount of time the CCD is allowed to collect light/photons to make a picture.
• Field of view - the amount of sky that you can see in your telescope or CCD camera.
• Image Scale - in astronomical images, the image scale refers to the angular distance covered by each pixel on the CCD.
• Image Size - the number of pixels across an image multiplied by the physical dimensions of each pixel. (Definition refers to an image recorded by a CCD camera.)
• Magnification - the number of times larger the image appears in a telescope compared to your unaided eye.
• Saturation - overexposing of individual pixels. Evidence of saturation can be found when a slice line through a star shows a slice graph with a flat top, or when one can see blooming of the signal from the saturated pixels to adjacent pixels.
• Resolution - the ability of a human eye, telescope or CCD camera to see detail or distinguish between closely spaced objects.

Preparation:

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.

Images

 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.

• 2-jupiter-a.fts Sufficient field of view to see all four Galilean moons. Jupiter is very tiny.
• 2-jupiter-b.fts Notice two of the moons. The next image was taken the same night.
• 2-jupiter-c.fts Notice three of the moons. This and the previous image together show all of the moons. A bigger field of view would be needed to see all the moons in one image.
• 2-jupiter-d.fts Out of focus, notice donut shape of moons.
• 2-jupiter-e.fts In focus.
• 2-jupiter-f.fts Three moons, where is the fourth?
• 2-jupiter-g.fts Detail fine enough to pick out the red spot. Tracking slightly imperfect, the moons are not pinpoints.
• 2-jupiter-h.fts Moons way out of focus. Notice mottled donut shapes.
• 2-jupiter-i.fts Good focus, notice bands on Jupiter and focused moons.
• 2-jupiter-j.fts Way out of focus, looks like a Cherrio instead of Jupiter.
• 2-jupiter-k.fts Jupiter and moons in excellent focus, field needed larger than CCD chip so camera was placed on an angle. Image is not oriented north/south.
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.
• 2-lagoon_nebula-a.fts Larger field of view but small image size
• 2-lagoon_nebula-b.fts Smaller field of view, greater image size, magnification, and resolution.

Moon

• 2-moon-a.fts Image taken with a SBIG ST7 CCD. The small pixels show detail. But, small pixel size and a small chip results in small field of view or small part of moon visible.
• 2-moon-b.fts Same telescope but different CCD. Same number of pixels as last image, but pixels are larger, and the CCD chip is larger chip. Whole moon can be seen, less detail on features.
• 2-moon-c.fts Apogee AP7p CCD, same as last image, but on a different telescope with a longer focal length and larger aperture. Notice larger image size and greater resolution.
• 2-moon-d.fts Apogee AP7p CCD but attached to a camera lens instead of a telescope. Note the smaller image size, lesser magnification and lower resolution.
• 2-moon-e.fts Image shows moon at a different phase. Different telescope and CCD system. More magnification than last image but not the resolution apparent in three of the other images.

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.

• 2-orion_nebula-a.fts Good focus, pinpoint stars, large image size, smaller field of view, some saturated stars, blooming.
• 2-orion_nebula-b.fts Very windy observing conditions, somewhat out of focus, larger field of view than image-a.
• 2-orion_nebula-c.fts Image size smaller, some stars saturated.
• 2-orion_nebula-d.fts CCD attached to a camera lens, very large field of view, smaller image size.
 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.

Time:
approximately 45-60 min.

Directions:

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.

Assessment/Evaluation:

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.