Investigating Astronomy logo
Home  |  About IA  |  For Families  |  For Teachers
Doing Science

Unit Four: Investigating Tools of Astronomy
Doing Science

1. Astronomical Image Processing: the Real Thing!

The experience you have had so far with astronomical images in the IA Data Center is only a taste of the variety of amazing things you can, and sometimes have to do, to images to derive the desired information. You have colorized single black-and-white images and changed the brightness of the image. In this "Doing Science," you will find out how the spectacular color images of galaxies and nebulae you see in Investigating Astronomy are made.

Color images on your computer monitor, TV, or camera film are made by combining three black-and-white images of the same object taken in different parts of the electromagnetic spectrum. Three images are needed because there are three types of color receptors in your eyes: one sensitive to blue, one to green, and one to red. To form a color image, one black-and-white image is colored blue, one green, and one red, and then all three are projected together. What black-and-white image goes in each color depends on what images you have available and what you are trying to reveal in the image.

First, gather at least three images of the same object. Here are a few sites where multi-spectral images are available:

  1. NASA's SkyView at: Go to the site and click on the "Advanced Interface." Once there, type in the name of an object (examples are provided), and select a Survey (for example, select 2MASS-J). Scroll down to the Optional Parameters and type in the desired values (use the defaults until you see what each does). Since you need three black-and-white images of the same size, type an image size in both pixels and degrees, and select the option "B-W Linear" for the color table. Once set for an object, leave the Options alone. Click "Submit Request Now." Once the image appears in a new window, click on FITS to download the image. Repeat for two or more additional Surveys such as 2MASS-H and 2MASS-Km to get your three images.
  2. The National Optical Astronomy Observatory "Pretty Picture Archive": Download at least three images of the same object.

Second, import your three images into an image processing program such as ImageJ or Photoshop. Adjust the brightness of each of the three images. In ImageJ, this is done by clicking on an image and selecting Image, then Adjust, and then Brightness/Contrast. A window with sliders will appear that you can move to change the contrast and brightness of the image. (Many astronomical images appear blank or dark when they are first imported, and must be "stretched" before anything appears.) Do the same with the other two images. Now combine the enhanced images into an RGB color image. In ImageJ, this is done by selecting Image, then Color, then RGB Merge. Select the image file for Red, Green, and Blue, click "Keep Source Images," and click OK. Your color composite image will appear. (Note: The process to make an RBG image will be the same in any image processing program though the names and locations of the commands will be different.)

Explore the effects of the colors by putting different files in Red, Green, and Blue, or change the enhancement of on or more of the black-and-white images before making the color image. If you have a fourth black-and-white image of the object, substitute the fourth file in different combinations with the other three to see how the color composite image changes.

Normally, different details in astronomical objects are revealed by different colors. For example, young hot stars are prominent in images taken in the blue; some gases are prominent in the red or green images; dust is prominently shown in far infrared images, and extremely hot objects are detailed in X-ray images. By changing the black-and-white images that go into the RGB composite image, the information the image shows changes also. Getting a "pretty" picture or an informative one is not always easy. Image processing is partly science, and partly art and obtaining a spectacular image usually takes a lot of trial and error.

Your Report:

  1. Obtain at least three black-and-white images of an astronomical object like a galaxy or a nebula. Create a color image of each.
  2. Make a PowerPoint or print report showing your image. Describe the image and what the different colors show. Explain why you chose that particular combination of images and colors to show your object.
  3. Repeat the process for a different type of object, or the same object in different parts of the electromagnetic spectrum. For example, if you used infrared "colors" like J, H, and K, find images made at visible wavelengths, like V, B, and R, to make a completely different color image of the same object. Describe and explain the differences.