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
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
- 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
- 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.
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
- 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
- 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
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-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
- 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
- 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.
- 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
- 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.
Notice rings of Saturn and moons of Saturn.
Compare image size to previous image. The magnification is much less
on this image.
Image is out of focus.
Image size is very small; resulting image of Saturn is tiny.
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.
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
Time: approximately 45-60
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
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
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.
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.
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
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