Teacher Page
Activity 6a Gathering Starlight, Experimenting with Aperture

 Standards (see Appendix A):

• Science Content Standards: A. Science as Inquiry, E. Science and Technology, G. Nature of Science.
• Mathematics Standards: Representation, Measurement, Connections.
• Technology Tools: Productivity; Research; Problem Solving and Decision Making.

Unifying Concepts:  Evidence, Models and Explanations; Systems; Form and Function.

Objective:  

Through this activity students learn the role of aperture size in the images produced by a telescope/CCD system.  By examining a series of images taken with the same telescope covered to provide different apertures, students are able to discover:

1. Images taken with larger apertures are brighter.
2. The brightness of images is proportional to the area of the aperture.
3. Aperture does not effect image size
4. You can see more stars on images with larger aperture
5. Very small aperture telescopes do not give clear images.

Overview:

Using HOU IP software, students compare images taken with the same telescope, but with varying amounts of the aperture covered.  Aperture is the term for the dimension of the main lens or mirror which is open to the sky.  Students make observations and gather data about these images and then draw conclusions based on their observations.  The Student Page for this experiment is quite open-ended.  Depending upon the class you are teaching, you may need to provide added guidance about the procedures and processes used to draw conclusions.

Background:

A common difficulty students have in understanding aperture is the concept that the light coming from a star is spread over a large area. This experiment is designed to encourage students to consider that although a star appears as a point, its light spreads out in all directions and travels long distances. If you and a friend who is standing some distance from you can both see the same star, you both are receiving a portion of the light from that star.  Your eye intercepts only a very small bit of the light coming from a distant star. Assisting students in imagining this scenario will help them to make the connection after completing the experiment that larger diameter telescopes collect more light in spite of the fact that the star looks very small.

The inverse square law of light describes the mathematical relationship between light intensity and distance from the source.  Although this activity does not address this aspect of light some good resources include:

• HOU Measuring Brightness Unit
• Exploratorium Inverse Square Law for Light Activity http://www.exploratorium.edu/snacks/inverse_square_law.html

Problem:

Students should understand that although telescopes have a number of different characteristics.  We are going to begin by exploring the effect of aperture. Aperture is the term for the dimension of the main lens or mirror which is open to the sky.  It is usually defined in terms of the diameter of the primary mirror or lens; however, as we will discover, it is the total area of this opening that is important.

The Experiment:

Since it is may not be possible for most students to take their own images, sets have been provided for them.  Be sure that students understand that in order to draw conclusions about how the size of the telescope opening affects images, all other factors must remain the same.  We change one variable at a time, the experimental variable.  Because seeing conditions (clouds or atmospheric turbulence) can affect image quality, taking all the images on the same night will help control this variable. 

Prediction/hypothesis:

Encourage your students to imagine the experiment of covering the part of the telescope open to the sky.  What will happen to the image if the telescope is completely covered?  How will the image change as more and more of the telescope is open to the sky?   Then be more specific.  In this experiment, the telescope was covered to leave openings or apertures of 2, 4, 6, 8, and 24 inches.  What will be the effect on the  images?  Students should write out their predictions and hypotheses before beginning the analysis.  If a star party is planned, have some cardboard and masking tape on hand to cover the telescope aperture partially so students can experience this effect while looking at bright and dim astronomical targets.

Image Sets: 6a-Aperture

Allow students or pairs to choose an image set or assign one of the image sets.  M56 is a globular cluster in the constellation, Lyra; M57 is a nicknamed the Ring Nebula and is a planetary nebula, also in Lyra. There are five images in each set, each taken with a different aperture of the telescope from 2 inches to 24 inches, as explained above. 

Analyze Images and Record Data:

Some of the observations students make about the images will be qualitative and some quantitative.  However, by using image processing and analysis tools, students will be able to quantify most of their observations about the images, organizing the data into tables and creating graphs.

Ideas for Analysis:

• Compare brightness data between images for individual objects.
• Calculate aperture area and compare to brightness data.*
• Compare the field of view for each of the images.
• Compare the size of individual objects.
• Use Data Tools, Find, to locate the number of stars at a particular brightness in each of the images
• Compare the Sky brightness.  If you plot sky brightness and aperture area the curve will be close to that found when you graph aperture area and the brightness of a particular star though it will not work as well.

*Spreadsheets can be used to share and analyze data.  See sample spreadsheet for M56 and spreadsheet for M57 data.  You may have to save these files to your computer and then open them with Excel.

Notes on HOU image processing software tools.  These can be found under the menu option Data Tools.

• Aperture – use this tool to compare the brightness counts of the same star on different images.  You find the tool in the menu for Data Tools, Aperture.  You may need to set or change the Radius and Sky Radius values; 7 and 14 are recommended respectively.   Radius defines the circle within which the pixels are recording the star's light. Sky Radius defines the circle where the software is sampling the 'sky' value.  The final result is a total of all the counts for the star minus what the software determines to be the average sky value for that region of the image.
• Auto aperture – measures the total brightness counts from individual stars; symbol is a bull's eye on the toolbar.  Auto aperture is likely to change the radii values from star to star. For this reason, the Aperture tool above is a better choice.
• Slice – measure the size of objects or the distance between objects
• Find - allows you to find groups of pixels which have the expected profile of a star and satisfy a minimum brightness level which you specify.  This tool might be helpful in analyzing the star cluster images rather then the nebula image.  (For m56 locating stars with a brightness of 100 counts above the sky works well).
• Sky – gives you a measure of the amount of light contributed from the sky as an average for the entire image or the portion selected.

Preparation: 

• Classroom or computer lab with HOU image processing software, 6a-Aperture image sets, and the internet if student page is to be viewed from the Internet source.  Post for the students the path to the folder containing the images.
• Make copies of Student Pages. Have graph paper available or student access to a program such as Excel for data analysis.

Time:  1 to 2 class periods.

Time for this activity will depend on students experience using image processing tools, plotting graphs, or using programs such as Excel.

Homework:   An optional homework assignment is provided that is designed to help students think about seeing starlight, reviewing what they know about aperture, and imagining the experiment before beginning to analyze the data. 

In Class:

Ask students to predict, analyze the images and explain the results.  See student page for specific directions.

1. Review the aperture experiment with students.  Be sure that students understand how the images were obtained and why the same telescope was used instead of different telescopes with the required apertures.  Emphasize that we attempted to change one thing at a time (isolating the variables) about the telescope/CCD in order to better understand how it works.  In this experiment we were changing the aperture opening by using a cover with various masks.  Using the same telescope but changing the size of its aperture, allows us to explore just the effects of aperture.  Students should predict what will happen to the images when the aperture of the telescope changes.
2. Ask the students to open all the images in a set and find the same star on each image in the set. Then students should use image analysis tools in the HOU-IP software to gather data. Encourage students to organize their data in tables, or spreadsheets.  This data is very useful in comparing diameter to area, and area to ‘counts’ for stars (using the aperture tool). Encourage students to do mathematical analysis and create graphs of their data. 
3. Create time for student groups to report and discuss the results of their analysis and how the results supported or did not support their predictions.

Evaluation/Assessment:       

Students should be able to return to the list developed during Activity 2 – Image Quality and add information about what causes the images to differ from one another in brightness.

Observe how students organize and display the data they collect from analyzing the images.  Are they able to construct graphs and interpret the graphs in light of the experiment.  Are they able to use the graphs to predict what would happen at various apertures that were not tested? 

At a star party, do students suggest partial covering of the aperture of a telescope to help provide comfortable visual viewing of a bright object such as the moon?

Refer to the Application section on the Student Page.