Turning Infrared Images into Representative Color Photos



A representative color RGB image of the Andromeda Galaxy (M31) with WISE 2 data in the blue channel, WISE 3 in the green channel, and WISE 4 in the red channel. The blue dots are probably field stars. The bright red areas indicate star-forming regions with lots of dust (heavy on the 22 micron wavelength) whereas the blue ares is heavy in the 4.6 micron wavelength.

 During fall semester, 2014, I taught the first half of a year-long astronomy course. This semester focused on constellations, cosmology, galaxies, and stars, whereas winter semester will focus on planetary science and the solar system.

M16-Lexi B-sharp-s

A representative infrared image of the Eagle Nebula, M16, by Lexi B. You can see the Eagle’s head and beak just to the lower left of the central red “finder” circle.

Because of my work at Brigham Young University for the Research Experiences for Teachers (RET) program during the previous summer, I wanted to incorporate what I had learned into my class by creating a series of new lesson plans. Ultimately, I wanted to experiment with these lessons, get student responses to them, and create a poster on their effectiveness to present at the American Astronomical Society conference in January. I would be going there anyway with the NITARP group, so why not present my own educational poster?

Helix Nebula-Wyatt-sharp-s

The Helix Nebula, a representative infrared image by Wyatt B. Notice the long infrared (22 micron) afterglow in the center of this planetary nebula and the shorter wavelength (3.4 and 12 micron) shock wave around it.

Altogether I worked up three lesson plans for this poster, including one on finding the distances to stars using the Distance Modulus formula (I have already published the precursor lesson about using the parallax method), another on using WISE infrared data to create representative RGB images, and a third on how to find data for stars and chart it into SEDs (Spectral Energy Distribution diagrams).

Here is a description of the second lesson. I am also attaching a finished PDF version here:

False Color RGB tutorial

Purpose: To teach high school astronomy students how to use the IRSA Finder Chart in the NASA/IPAC database to locate and download infrared data files from WISE and other missions, then use IR images of various wavelengths to create representative RGB images.

Part 1: Using IRSA to Locate and Download Infrared Images

IRSA is the NASA Infrared Science Archive located at IPAC, the Infrared Processing and Analysis Center at Caltech in Pasadena, CA. Infrared data and images from all United States and some other missions are archived at this location. The URL link is:


Missions that have archived data here include WISE, Spitzer, IRAS, 2MASS, Herschel, Planck, Akari, and others. It is your one-stop shopping center for infrared data.

IRSA website

The main webpage for the NASA/IPAC Infrared Science Archive (IRSA). To download WISE or other data, click on the Finder Chart button.

To use IRSA, click on the “Finder Chart” icon/button from the homepage and it will get you to the search engine. You will need to type in the name of the object or its coordinates (right ascension and declination or galactic coordinates), then choose the area of the sky to look at, ranging from degrees down to arcseconds. Then choose the display size (usually you want “large”) and the datasets to retrieve, such as DSS, SDSS, 2MASS, WISE, or IRAS. Then click “Search.”

IRSA search entry

The Finder Chart search engine. Type in the name of the object (it may need to be a technical name, such as Messier 31, instead of the colloquial name) or you can type in the coordinates for the area you wish to view. You can specify which catalogs, such as WISE and 2MASS, and how large of an area of sky, down to 300 arcseconds.

For example, let’s say you want to look up the Horeshead Nebula. If you type that into Finder Chart, then it will tell you it can’t resolve the name. So you would want to find a more scientific name, possibly in Wikipedia. You would find it is also called Barnard 33 or emission nebula IC 434. Now the search engine is able to resolve the name (in other words, it found the data).

Horsey WISE and IRAS

The results for the Horsehead Nebula (IC —) in the IRSA finder chart search. This shows that WISE 1-4 wavelengths as well as the IRAS wavelengths (12, 25, 60, and 100 microns). The WISE mission had much greater resolution.

Using the default size of 300 arcseconds produces only a small part of the nebula, so you will need to increase the area covered by the image, let’s say to 20 arcminutes. We now get good images from DSS, nothing from SDSS, just small images from 2MASS, nicely detailed images from WISE, and very pixilated images from IRAS. You can tell from these that IRAS had much lower spatial resolution than the other space probes, with WISE and DSS having the best.

Horsey RGB-s

The Horsehead Nebula in infrared wavelengths. Notice the bright red star at the top of the horse’s head, which is invisible in the true color image below.

To download the images, click on the Save button (which looks like a floppy disk) in the upper left corner and choose the file type. For using these in Photoshop or Gimp, you will want to choose PNG for the format. If you will be using DS9, then choose FITS format. When the PNG file pops up in your Preview window, you will need to resave it under a better name, such as the name of the object and which mission/wavelength the image is, such as WISE 4 or 2MASS H band. Save them in a folder other than the Downloads folder for easier access.

Horsey true color

The Horsehead Nebula in true color. The dark nebula hides a hot, young protostar that shows up nicely in the WISE image above. The dark wall below in this image becomes a glowing cloud in infrared.

This lesson gives instructions using the menus and commands in Adobe Photoshop, but you can use GIMP instead, an open source program that is free for download. The instructions/commands are similar.

How Computers Handle Color – And What is Meant by a “Representative” Color Image:

Computer images are made up of three “channels,” which are images made of 256 shades of gray (in 8 bit color, which is 2^8 colors or 256). Each channel is an additive primary color: red, green, or blue. If you don’t know what I mean by an additive color, please do a Google search and look it up. All I want to say here is that color on a printed page is subtractive – as you add more pigments, the image gets darker as more light is subtracted. The primary colors are the colors of pigments: cyan, magenta, yellow, and black. But the image on a computer screen adds light to light, and so the primary colors are red, green, and blue. Red and green together make yellow. All three together make white.

Pasting in Green channel-s

The channels palette in Adobe Photoshop. By selecting and copying a narrowband image (say WISE 3 at 12 microns) and pasting it into only one channel (here the green one), three separate narrowband wavelengths can be built into one representative color RGB image.

The process here is to take three 256 grayscale infrared images at increasing wavelengths (such as WISE 1, 3 and 4) and use them to replace the blue, green, and red wavelengths respectively. The final image is not true color but represents the original invisible infrared wavelengths with colors our eyes can see.

Part 2: Combining Images in RGB

You will want to open Adobe Photoshop and choose “File-Open” and open three of the four WISE images or all three of the 2MASS bands. For WISE data, I recommend either the WISE 1 or 2 (3.4 or 4.6 microns) for the blue channel, WISE 3 (12 microns) for the green channel, and WISE 4 (22 microns) for the red channel.

Start with the WISE 1 or 2 image as your starting point. Choose “Image-Mode” and convert the photograph to RGB with 8 bits per channel. Then click on the WISE 3 image and select all of it (Command-A), then copy it (Command-C). Go back to the WISE 1 or 2 image and open the Channels window. Click on the green channel only, and paste in the WISE 3 image (Command –V). The WISE 3 image should now only appear as a grayscale image in the Green channel of the WISE 1 or 2 RGB document.

All channels in-negative

All three of the wavelengths are now combined as separate channels. However, since astronomical images are usually inverted (space is white and stars are black), we have to invert the image here.

Click on the WISE 4 image, select all of it and copy it, then go back to the WISE 1 or 2 image, go to Channels, select the red channel only, and paste in the WISE 4 data.

This will give an RGB image with the three wavelengths superimposed as the blue (WISE 1 or 2), green (WISE 3), and red (WISE 4) channels. This image will be inverted, so go to “Image – Invert Image” to have black as the background space color.

You may want to make some lighting adjustments (Image – Adjustments – Levels) and increase the resolution and dimensions of the image (Image – Image Size).

All channels postive

All the channels combined and the colors inverted so space is black. This is an image for one of our HG-WELS stars, where the IRAS data had source confusion (a grouping of stars “spoofed” the IRAS sensors). The actual target K-giant star is the one to the left of the red finder circle.

Save and rename the image so you know it is RGB. Then pat yourself on the back. You’ve done it. Or better yet, do another one.

m16eagle-Dave M-enhanced-s

Another view of the Eagle Nebula, M16, in representative infrared colors, this time with a larger view area. This was done by Dave M.

About davidvblack

I teach courses in multimedia, 3D animation, Earth science, physics, biology, 8th grade science, chemistry, astronomy, engineering design, STEAM, and computer science in Utah. I've won numerous awards as an educator and am a frequent presenter at state and national educator conferences. I am part of the Teachers for Global Classrooms program through the U.S. Department of State and traveled to Indonesia in the summer of 2017 as an education ambassador. I learned of the Indonesian education system and taught classes in astronomy and chemistry at a high school near Banjarmasin in southern Borneo. I am passionate about STEAM education (Science, Technology, Engineering, Arts, and Mathematics); science history; photography; graphic design; 3D animation; and video production. This Spaced-Out Classroom blog is for sharing lessons and activities my students have done in astronomy. The Elements Unearthed project (http://elementsunearthed.com) will combine my interests to document the discovery, history, sources, uses, mining, refining, and hazards of the chemical elements.
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