# Investigation 2: Light and Color - Activity 6

## "True" X-ray Color Image of Orion

Overview: Students learn to make a "true color" image of Orion in X-ray light.

Electronic resources: Orion Chandra image (X-ray): (OBSID 3744, available through the ds9 Virtual Observatory, Process: How to open a certain OBSID).

Lecture: How X-ray detectors are different from visible light.

• Visible light detector: records # of counts and position only, must collect energy info using filters or filter the image before we take it, then combine RGB filtered images for true color.
• X-ray light detector: records # of counts, position and energy! so, we can "filter" the image after we take it.
• Bottom line is that X-ray detectors record position and energy for each photon, whereas visible light detectors only record position for each photon collected.

Create a true-color X-ray image of Orion:

• We cannot see X-ray "colors" (energies), but we can use color to represent the energy of each photon, assigning red to low energy photons, green to medium energy photons, blue to high energy photons.
• Instructors demonstrate how to make energy cuts on images with low energy cut. (Process: energy cut for X-ray image)
• Students repeat with medium and high energy cuts.
• Instructors show how to combine 3 color image. (Process: Creating a true color image by opening RGB frame.)
• Instructors show how to measure counts in images, put into a chart, again looking for patterns (should see that red objects have the highest flux in high energy filtered images, and blue objects have lowest flux in low energy filtered image)
• Students open visible light image of Orion, and use color tables to add color, then match the frames and crosshairs. (Process: Match frames, Process: Lock crosshairs)
• Focus question for students: "Compare your two images side by side. How did we use color differently in each of these images? For each image, choose two different colors, and describe what each of those colors represents." Setup of images to allow students to compare: (true color/false color)
• Wrap up discussion: Why do we call things "true color" and "false color"? Which is easier to make? Why?

Teacher tips and tricks:

• It may be useful to have students read about the capabilities of Chandra before discussing the differences between X-ray and visible light true color images, so they learn about recording energies with the CCD detector.

Assessment ideas:

• Show two images, explain how color is used differently in the two images. (One false color, one true color)
• How could you tell if something is true color or not? False color if centers of all point sources are same color, because that indicates some maximum brightness. True color if point sources are different colors.
• Have students define true color and false color using their own words, but require them to use the words flux and energy of photons.

Note: Investigation 2, Activity 6 begins around 09:00.

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