Activity 7 Experimenting with Exposure Time
Standards (see Appendix A):
- Science Content Standards: A. Science as Inquiry, E. Science and Technology, G. History and Nature of Science.
- Mathematics Standards: Representation, Measurement, Connections.
- Technology Tools: Productivity; Problem Solving and Decision Making.
Unifying Concepts: Evidence, Models and Explanation.
Hubbles Variable Nebula, NGC 2261 in Monoceros. Compare the two images, on the left the exposure time is 10 seconds, on the right 60 seconds. What differences do you see? What differences can you measure?
Students build on an intuitive knowledge of exposure as the amount of time light is collected for an image. Students learn that in the CCD there are rows and columns of pixels where the energy from photons is collected as electrical charge and counted. These counts are proportional to the brightness of the celestial object and the length of time the CCD is exposed to the light from the object. Students investigate image sets, comparing counts with exposure time. Through this investigation, students learn that manipulating the exposure time of images can be used to enhance or saturate detail in astronomical targets without affecting magnification and field of view. Students should reach the conclusion that in addition to aperture of the telescope, one can manage the amount of light collected for an image through exposure time.
Students use HOU IP software to analyze several different image sets to determine what effect changing exposure time has on telescopic images. Several different sets are available so that students can make comparisons between several types of objects. All image sets were taken using the Yerkes 24 inch reflecting telescope; images within each image set were taken at approximately the same time minimizing the effect of changing seeing conditions.
If your students have worked through the previous experiment, Activity 6a Effects of Aperture, they should have sufficient experience to follow the procedures and come to conclusions using the student pages as a guide. If needed, however, homework and conclusion sheets have been provided for you to use. As always, the level of sophistication your students use in gathering and analyzing their data is largely a function of their level of experience using the HOU-IP software and conducting experiments. Information in the Teacher Background section provides additional information should you need to assist your students in using new tools and techniques.
Think Before You Experiment
- In visual astronomy, the ultimate detectors and interpreters of the light are the human eye and brain. The eye senses the light and sends continuous signals to the brain to be interpreted as visual images. Unlike the eye however, a CCD camera allows us to vary exposure time, and save the information digitally.
- Consider the analogy of a telescope as a light funnel that captures and concentrates (focuses) light. In this analogy, we need a way to collect and store the light coming down the funnel. Just as actual funnels require a separate container to deposit the liquid, once a telescope has gathered and focused the light we need a detector for that light.
- Think of the CCD as the bucket at the end of the funnel. Better yet, imagine the array of pixels in a CCD as an array of buckets. And like a bucket, each pixel has a set capacity. If it is filled beyond this capacity, light will spill over like water into the surrounding pixels. The amount of time the pixels of the CCD are allowed to fill with light energy is called exposure time. At the end of the exposure time, the buckets are emptied; the energy is counted. The information is saved as a digital file, later to be opened, displayed and analyzed on a computer using image analysis software.
- Students compare images of varying exposure time to discover the relationship between exposure time and brightness of an image. Once an understanding of exposure time has been built, students can consider how changes in exposure time might affect astronomical images.
- Reinforce once again the experimental nature of what they are doing. We are comparing images for which we have tried to hold all variables constant, changing only the exposure time for each.
- Because all the images were taken with the same telescope at the same location under similar seeing conditions, most variables have been held constant. The focal length and aperture are the same. The same CCD camera and filters were used for each image set.Only the exposure times are different. Although seeing conditions can change between exposures, these images have been selected for their consistency. Varying clouds and atmospheric conditions should be considered as uncontrolled variables when considering possible sources of error.
- What will happen to an image when a CCD is exposed twice or three times as long? What will happen to star images, what will happen to more diffuse objects such as nebulae or galaxies?
- Note: One of the attributes of CCD imagers is they are more efficient detectors and collectors of light. CCDs differ from traditional photography in that their ability to collect light does not drop off with exposure time as happens with film and traditional photographic methods.
- Alcyone_time: Alcyone is the brightest star in the Pleiades star cluster. This star cluster is in the constellation, Taurus. Longer exposures show affects of saturation and blooming. Saturation occurs when the pixels which collect a stars light are all filled to capacity. If a student slices the star, it will have a flat top instead of a peak in the plot of the slice. Another affect is blooming; too much light can cause pixels to overflow into other pixels. This is called blooming.
- Bubble_time: NGC 7635 , the Bubble Nebula. Two of the images are one minute, but are centered differently. One image is three minutes. Some stars are saturated. You can see another effect of exposure time is that the telescope may not be tracking the sky perfectly and there is elongation of star images or trailing. This is due to a mismatch between the rate the telescope is moving and the rate of Earths rotation. Different parts of the celestial sphere appear to move at different rates, the equator being the fastest, the poles the slowest. See this time exposure of the sky looking towards Polaris to consider how the sky appears to move faster as one moves away from Polaris.
- Crab_time: M1, the Crab Nebula. It is in the constellation Taurus. Pick a spot in the nebula and compare the pixel brightness counts of the three exposures. Pick a star and compare auto aperture counts.
- Galaxy_time: Set includes several galaxies. Galaxies generally have low surface brightness. It is difficult to observe them visually, because they appear fuzzy and only slightly brighter that the sky. Exposure time helps us collect more light, seeing features such as spiral arms with longer exposures.
- NGC 2535 in Cancer
- NGC 2903 in Leo
- NGC 6503 in Draco
- NGC 7331 in Pegasus
- Glob_time_m2: M2, globular cluster in Aquarius. This set and other globular cluster sets show how longer exposure times show more stars but also make it more difficult to separate bright stars in the central regions of the clusters.
- Glob_time_m3: M3, globular cluster in Canes Venatici.
- Glob_time_m71: M71, globular cluster in Sagitta.
- Hubble_time: NGC 2261, variable nebula in Monoceros. This nebula was found to vary in brightness by Edwin Hubble during his graduate studies at Yerkes Observatory. A photographic glass plate which was taken of this nebula by Hubble was recently flown on the Space Shuttle by astronaut John Grunsfeld in commemoration of Edwin Hubble.
- Open_time: NGC 7331, an open cluster in Pegasus. This image was taken at two different exposure times, 40 seconds and 120 seconds, on Fathers Day in 2001. (In honor of Edward Gaiden) So, the unofficial nickname of this cluster is Fathers Day Cluster! Notice the nebulosity which becomes much more apparent with the longer exposure.
Ring_time: M57, planetary nebula in Lyra.
- Stars_time: Random star field.
** Save the images in the your_time folder for the evaluation section.
Analyze the Images and Record Data
A CCD camera has a detector inside that is made up of a grid of pixels (picture elements). When light such as starlight falls on the CCD, the energy from the photons of light is counted electronically. The counts recorded with the CCD are the Brightness Counts that you see when you move your cursor across an image in HOU-IP software. So, these counts represent the amount of light that was sensed by the CCD detector. HOU-IP software has color palettes and display tools that change how the picture looks without changing the counts in each pixel. For example, the counts for the pixels stay the same no matter how you change Min/Max or if you have LOG scale checked or not. In the menu of the HOU software, there is a pull down menu called Data Tools. Data Tools help you analyze the image data represented by counts in each pixel. One tool is the Aperture tool that adds together all the counts associated with a star but subtract the counts that belong to the background sky. Slice takes a line you draw across an image and represents counts in each pixel crossed as a position on a graph, at the same time counting the number of pixels on the line as a distance (in pixels). Another pull down menu is Manipulation; some of these options allow you to do mathematical operations to all the counts in an image. When you use mathematical manipulation tools, you are altering the counts, which is the original data collected by the CCD. There are other manipulation tools for shifting the counts from all the pixels up or over in an image, or for rotating or flipping the image; these change the position of pixels in the image and their corresponding counts.
Analysis options to consider:
Compare brightness data between images for individual objects.
Compare the brightness of the sky between images.
Compare the field of view for each image.
Count the number of stars that are visible at different min/max settings.
Compare the size of objects between images.
Make qualitative statements about the detail emphasized for different targets.
Suggested HOU tools:
- Auto aperture measure the total brightness counts from individual stars
- Sky measures the contribution to the total brightness of the image coming from the background
- Subtract allows you to subtract the brightness counts contributed from the sky for each image
- Slice slicing across a star will reveal a flattened top if the star is over-exposed
- Zoom Box zooming in on individual stars can assist you in evaluating the focus quality and exposure of the image. Blooming is emphasized as well as the compactness of the star itself.
Conclusions Using these images, students should be able to discover:
- Brightness increases with exposure time.
- Brightness of individual objects and the sky increases proportionally to the exposure time.
- Field of view is not affected when exposure time is changed.
- Magnification is not affected when exposure time is changed.
- The number of stars visible increases with increased exposure time.
- Detail in brighter areas is better with lower exposure times.
- Detail in dim areas is better with increased exposure times.
- Overexposed stars exhibit a streaking pattern called blooming resulting from photon signal spilling into adjacent pixels. This is usually forms a vertical or horizontal pattern depending upon characteristics of the CCD chip and positioning of the camera.
- When considering the exposure time for a particular object you need to consider what it is you want to study. Brighter is not always better.
- Be sure the image sets needed for this experiment are available to your students
- Make copies of Student Worksheet
- Make copies of Homework and Conclusion worksheet if needed
The time needed to complete this experiment will vary depending on the structure you decide upon and the amount of guidance you decide to give groups in data gathering and analysis. Having all groups examine all the image sets but gather data on only one will save time but narrow the scope of the conclusions somewhat. If you expect your students to gather and process quantitative as well as qualitative data, allow additional time for data gathering.
- Introduction 15 minutes
- Data Gathering - 50 minutes
- Options for Analysis optional
- Discussion 20-30 minutes
Review the experiment with students. A variety of celestial objects were imaged at varying exposure times. Emphasize that the experiment was too change just one thing at a time about the telescope/CCD system for each set of images. That makes time the variable in each set of images. Ask students to predict the effect they think exposure time will have on the images. How will they measure the effect they predict? Students should also predict what will not change as a result of varying exposure times.
After the students have worked with one or more sets of images, ask the class to summarize the results.
- Students should be able to return to the images and their lists of observations about the images of Activity 2-Image Quality. Having completed this activity and the previous activities regarding focal length and aperture, they should be able to consider what may have caused the images to differ from one another with more understanding.
- The conclusion section of the experiment is intended to act as the evaluation for this experiment. If students are able to express what they learned, document what they know about the effects of aperture and how they know it, then they have been successful in accomplishing the objectives for this activity.
- In addition, if you had groups of students concentrate on a particular set of images, you may ask them to look at a different set of images, suggest one or more analysis options for that set and explain what aspect of exposure time they best exemplify.
- Using the images in the your_time folder, ask students to construct a graph of brightness counts verses exposure time for a star of their choice. Students should be able to write conclusions from their graph as well as making qualitative statements concerning the exposure time and quality of the images in this set.