Coloring the Universe Teacher Guide

Introduction

Nearly everything we know in astronomy comes from the light we receive from astronomical objects. Fortunately, that light provides a wealth of information.

The Rubin Observatory LSST Camera is the most sensitive camera ever built for astronomy, and uses six filters to observe the Universe from ultraviolet through infrared wavelengths. Color astronomy pictures are assembled from images obtained through these filters. To the casual observer they may simply look pleasing, but to those skilled at interpreting them, they provide valuable scientific insight.

This investigation combines science, technology and human creativity (STEAM). Students will learn about the function and benefits of filters, the technology of digital imaging, and how to construct chromatically-correct color images that tell a scientific story.

Learning Outcomes

• Students can explain how filters work: that light is not “changed” to a certain color by a filter, but that some wavelengths of light are blocked by a filter while others are transmitted.
• Students can give an example of how astronomers use infrared filters in astronomical cameras.
• Students can create and use a representative color image to demonstrate how color-coded visual information is used by astronomers to learn about an object.
• Students can assign a correct sequence of colors to wavelengths using the chromatic ordering technique.
• Students can explain the process by which a digital color image is made, including the use of filters, detection of light at the image sensor, colorizing and combining images, and adjusting the relative intensities of colors.

Prerequisite Concepts

• Students can recognize and order different types of electromagnetic radiation by their relative frequencies and wavelengths.

Where This Fits In Your Teaching

• electromagnetic spectrum, properties of light and color
• the technology of telescopes, filters, cameras and digital images
• creating color astronomical images / image processing
• interpreting astronomical phenomena with colorized images (distant galaxies, star forming regions, nebulae)
• multiwavelength astronomy
• STEAM projects: a blend of science and art
  

NGSS Storylines

• How do filters work?
• How are color digital images made?
• Why do scientists study an object using multiple wavelengths of the electromagnetic spectrum?
• What is chromatic ordering and why is it used?
• How can filters be used to:
  • Identify star-forming regions?
  • Estimate temperatures of stars?
  • Identify far away galaxies?
  • See through dust?

Suggested investigations which could come AFTER this one:

Exploring the Observable Universe

The Expanding Universe

A Window to the Stars

See Related Rubin Observatory Investigations for more details.

Investigation Timing

Online component: 1- 2 hours

Standards

Three-dimensional lesson summary

Students will learn about the function and benefits of filters, the technology of digital imaging, and how to construct chromatically-correct color images that tell a scientific story.

Students use digital imaging technology to observe astronomical objects at different wavelengths, then design a process to create an image by choosing filters, colors, and adjusting intensities and communicate information about the object's composition or distance.

Building towards:

HS-PS4-5 Communicate technical information about how some technological devices use the principles of wave behavior and wave interactions with matter to transmit and capture information and energy.

HS-ESS1-2 Construct an explanation of the Big Bang theory based on astronomical evidence of light spectra, motion of distant galaxies, and composition of matter in the universe.

HS-ESS1-3 Communicate scientific ideas about the way stars, over their life cycle, produce elements.

Science Literacy and Critical Thinking Skills

• Analyzing and interpreting data
• Obtaining, evaluating & communicating information

Background

The image detector of a digital camera, whether in your phone or in an astronomical instrument, contains an array of light receptors that only detect levels of light, not color. In order to convert a camera’s greyscale image into a color image, filters are used to allow the detector to observe only a specific range of wavelengths, then colors are assigned to them.

Astronomical images taken through broadband filters are typically constructed using the chromatic ordering technique, in which the light from each filter is assigned a representative color. The light transmitted by the filter passing the shortest wavelengths is assigned the color of the shortest wavelength of visible light used to make the color image, and the process continues with each successive filter, so that the light passing through the filter that transmits the longest wavelengths of light is assigned the visible light color with the longest wavelength. Images constructed in this way are known as representative color images.

Openstax Astronomy textbook links:
Visible light detectors and instruments
Electromagnetic Spectrum

Links to Videos

These resources are from a teacher workshop on this investigation.

Video: Travis Rector, "Coloring the Universe"
Speaker slides (PowerPoint download)

Teacher Notes

1. Students may not feel confident about how to construct a “correct” six color image. You may hear questions like, “What should it look like?” Reassure students that there is no one “right way” for the image to look, since human eyes only use three colors and the Rubin Observatory LSST Camera has the superhuman ability to see in six. However, there are wrong ways to construct an image. Colors should be assigned to filters by using chromatic ordering. Sliders should be adjusted for each filter so that the image shows the contributions of each color used. In the end students should attempt to make an image that’s aesthetically pleasing and communicates the desired science information.

2. The discussion about how filters pass light in the introduction (and in the filter tool) is intentionally oversimplified. Filters are often designed to pass a range of colors, not just one. For instance, an orange filter may pass red and yellow light too. Each LSST Camera filter passes a broad range of wavelengths. Note, for example, that the g filter passes light from violet to yellow wavelengths.

3. Students may question why the first practice image is constructed using red, green, and blue colors. Some may have had prior classes or experiences working with ink or pigments, where the primary colors are yellow, cyan and magenta.

   Mixing pigments or dyes uses a subtractive process to produce color, whereas mixing light (which is what this investigation is about) relies on an additive process to mix colors.
   
   The human eye has three types of color receptors (cone cells) that work together using an additive process to mix light so that any color can be interpreted by the brain, in the same way that red, green, and blue light can be added together to produce any color of light. Since cameras use and mix light to produce images, red, green, and blue filters are the typical filters used.

4. Not all available filters need to be used to construct a color image. Students can choose to turn off a filter if it doesn’t contribute to the desired result.

5. Some students may notice that the figure showing the color filter pattern over the image sensor contains twice as many green filters as red or blue. This common arrangement is known as a Bayer filter, and is designed to mimic the physiological response of the human eye, which is most sensitive to green light.

6. This activity concludes with a choice of assessment tasks:

   • identifying far away galaxies
   • using infrared filters to see through dust
   • identifying young stars and star-forming regions in galaxies

   
   

   It may be useful to have a discussion with students after they have completed the practice six color image to clarify which options are available for further study. When students have decided on their object and question, they should draft a plan (as described in the investigation) for which filters and images they will need and how they will construct and analyze their image(s).

7. In the user-testing version of this investigation, some images have only 5 filters. In that case, the missing filter will appear to be greyed-out.

8. The six-color mixing tool relies on the use of broadband filters, so students may not get the expected result when adjusting the intensity of a filter. For example, increasing the intensity of the r filter adds more green, yellow, orange and red light.
   

9. Cyan and magenta have been provided in the testing version of this investigation as assists for color impaired users. If you get questions about the correct chromatic order for these colors, cyan is a recognized pure spectral color with wavelengths between blue and green (485-500 nm). Magenta, however, is not a pure spectral color, so we advise against non-color impaired students choosing it. Since magenta is a mixture of red and violet, it could be used in place of a red for those with red-green color blindness. Cyan could be used in place of blue or green.
   

Common Student Ideas

1. When astronomers use a telescope and camera to take an image, a color image automatically appears.

   Bridge to learning: The investigation leads students through the process of how certain filters are selected and placed in front of a detector (camera), which can only produce a greyscale image. The resulting images are then combined to create a color image. As students use the color mixing tools in the investigation, they will develop an understanding of how astronomers select, combine, and “colorize” the light from different filters, and adjust levels to create a color balanced image.
   

2. Filters change the color of light.

   Bridge to learning: The investigation leads students through a tutorial using an interactive filter changer. This helps them to visualize that certain wavelengths of white light are blocked by a filter while other wavelengths are permitted to pass.

Common Student Questions

1. There are red stars in the nebula image that were not hidden by dust. Why are they red?

   Some stars are just red in color. By comparing images, it’s easy to see which of the red stars are not visible in other wavelengths of light because they were obscured by the gas or dust.

2. Why are there gaps in the Rubin Observatory LSST Camera filter ranges?
   

   These omitted wavelengths (such as from 923-969 nm) are absorbed by Earth’s atmosphere, so very little radiation reaches the ground.

Differentiation

Students practice with interactive tools to gain an understanding of how filters work and how color images are constructed. They have the opportunity to evaluate their results after recreating an image in three colors before proceeding to the more complicated task of assembling an image in six colors. They also practice with constructing correct chromatic ordering sequences.

This investigation is an excellent cross-curricular STEAM project. Students can incorporate skills they may have gained from art, photography, or graphic design classes.

The six color mixing tool offers students a great degree of creative freedom in the selection of an image, the choices and ordering of colors, and in each color’s adjustment.

The assessment task also empowers students to make choices that suit their interests and skill sets. The large degree of freedom (topic, number of filters, color choices) also provides a wide range of challenge, from constructing one image that uses only two or three filters, to creating more complex images that require the use of five or six filters. Additionally, some students may choose to construct and examine multiple images to compare and contrast their properties. Using multiple images may lend itself well to group work, in which each student is responsible for the construction of one image and the group works together to determine the results. Plan for additional time if you intend to have students investigate higher-order questions.

Beyond making pretty pictures, the narrative component of the assessment task compels students to show how the evidence in their image(s) answers the science question they chose to investigate. A step by step guide is provided in the investigation to aid in the construction of their presentation or narrative.

More advanced options are provided in the Ideas for Further Study section of this teacher guide.

Related Rubin Observatory Investigations

• In Exploring the Observable Universe students compare galaxy redshifts in the early Universe with more recent times, to explore the Universe we can see and to observe how the large scale structure of the Universe has changed over time.
• In Expanding Universe, students use galaxy redshifts in the Hubble-LeMaitre Law as a way to investigate the expansion and acceleration of the Universe.
• In "A Window to the Stars", students use H-R Diagrams and histograms to explore properties of stars, such as color, temperature, brightness, lifespan and mass.

Ideas for Further Study

• Examine the spatial distribution of star-forming regions or hot stars in different types of galaxies or interacting galaxies.

• Build the same image using different (correct) chromatic ordering schemes to determine if there is an optimal way for displaying information.

• Students can create a three color (red green blue) light mixing device (this could be as simple as using flashlights with three colors of cellophane attached to them). They can create a chart showing which colors are produced from combinations of two lights, then all three. Next, have them experiment with color mixing six colors, such as with Rubin Observatory images. They may create and complete a similar chart with all six spectral colors.

  What challenges are associated with going from the RGB color mixer to a six color mixer? What advantages does the six color device have over the three color device?

  For an even more advanced challenge, students can construct a color mixer with adjustable levels of brightness for each color (dimmable LED lights would work well).