Combining Images to Make a True Color Image


The 3 images obtained through the different filters are now ready to combine into a 'True Color' image. The HOU-IDL software will be used in this activity to combine the red, green (visual), and Blue images into one 'True Color' image.


This activity will allow you to combine your three images taken through 3 different color filters into one new color image. Essentially, the 3 images will be digitally combined into one new image. The main difference is that the new image will not have one count value assigned to each pixel as in a typical FITS image, but 3 different count values. One count value will cause the computer to display a certain amount of blue, a second count value displays a certain amount of red and the third count value assigns a certain amount of green. Does this mean our image will only be 3 colors? We are all familiar with the idea that if we mix two colors of paint together, our eyes will see a new color that is a blend of the other two. Similarly, if we combine two colors of light, our eyes will see light of a third color. The same idea holds for our images. On the new image, a pixel that has a large blue count, a large red count, and a small green count would appear purple. A pixel that has a large green count, a large red count, and a small blue count would appear yellow. Therefore, we can have an image that begins as only 3 colors, but shows all the colors of the rainbow by different blends of these 3 colors.


Reminder: To proceed into the next steps of this process, you should have:

a. Measured and noted differences between the 3 images of your object taken through different filters.

b. Subtracted the SKY count values from EACH image.

c. Corrected EACH image for camera sensitivity and filter transmission.

d. Aligned the 3 images.


1. Open HOU-IDL Level 3 as well as the three alaigned images from the previous activity.

2. Process the images for the best view. Try to adjust the images so all three have about the same appearance to the background. This is important to the final outcome of your color image.

3. Use the 'Image Comparison: True Color' tool. When it opens, you will see drop-down menus to assign each of the 3 images to the correct color. Choose the correct images for each color. Inspect the image in the preview window. You can move the sliders to correct the Min-Max values for each color separately. Use this if necessary, but remember the bigger the changes you make, the further away from 'True' color the image will become.

4. Save your final image as a FITS file and as a JPEG. Use the 'File: Save As' tool to name the file appropriately and set the file type.


Students can compare images between other classmates and other resources such as Hubble or Chandra. If multiple students used the same image set, varying outcomes can be used to generate discussion. The importance of knowing the source of an astronomical image, what wavelengths it covers, and how color is used to represent different wavelengths are all topics which may be explored. The politics of scientific research funding can even be discussed in light of the flashy colors used by image processors for Hubble or Chandra.