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.
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?
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:
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.
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.”
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).
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.
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.
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.
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.
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).
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.