Astro-Images | Making Color Images

Out In Space

Some photographers have long telephoto lenses which allow them to create highly magnified views of small objects like comets. Others attached their camera body directly to their telescope. These days, their cameras have a CCD array instead of film. The CCD is able to capture color images using red, green, and blue light. The electronic signals are mixed in the camera to produce the colors we see in the photographs. Most of the images available for Comet Lovejoy at are in color. The comet appears as a faint blue-green object like this fine example.

Wolfgang Jörg | January 13, 2015 | Vienna, Austria

CCD devices are also attached to the large telescopes of the major observatories of the world. The Hubble Space Telescope is also equipped with them. The CCD arrays normally capture grayscale light in a wide range of shades, not in full color. These allow for a much broader range of sensitivity than film. They also allow finer detail, or resolution, of the image than a color CCD. The technical details are many and complex.

Basically, in order to make color images of objects in space, these telescopes capture 3 images of the object. Each image is made by placing a red filter, then green, then blue, in the light path before it reaches the grayscale CCD. The 3 color-filtered images are later combined with software to make a color composite by technicians on the ground.

I recently requested three images from the Rigel Iowa Robotic Telescope in Arizona operated by the University of Iowa Physics and Astronomy Dept. for research and education. It operates with a black and white CCD. I had options in the request form of how much time of exposure and what filter I wanted for each image. I chose 120 seconds and a red, then green, then blue filter for each of the three.

Here they are after adjusting the minimum and maximum values of the range of exposure for each. They have been cropped to this small size of 486 x 486 pixels for illustration in this post. Normally, they are 2048 x 2048 pixels wide and tall and are 8 megabytes each. The telescope tracked the comet. Notice the short star trails resulting from the 120 sec exposures.

Red filter used

Red filter used

Green filter used

Green filter used

Blue filter used

Blue filter used

The next morning, I received an email telling me my images were ready. I downloaded each from the observatory server. Using free ImageJ software, I opened each and adjusted their exposure ranges. Then, I chose a command to Merge Images in the software. This composite below immediately appeared on screen. I zoomed in a little more to show the detail of the 120 sec star trails with red, green, and blue streaks aligned end-to-end. The comet halo ended up the expected blue-green color. I felt quite successful.

Composite of 3 RGB images

Composite of 3 RGB images

Now, I want to download some 3-color filtered images from the Hubble site, or another major telescope, and try my hand at bigger and better. The images are publicly available…our tax dollars at work. Who knows? Maybe I can do something like this someday…one of many on Astronomy Picture of the Day.

Astronomy Picture of the Day | Light Echoes from V838 Mon | NASA, ESA, H. E. Bond (STScI)


Closer To Home

I wanted to try the 3-color technique on something here at home. Melanie is a skilled and talented quilter. Visit her Catbird Quilt Studio blog if you like quilts. Look in the galleries for beautiful examples. We have this quilt hanging on our living room wall.


Click to embiggen

I placed my camera on the tripod and set it to take black and white images instead of color. I have a set of color transparencies that include red, green, and blue. I set the self-timer on the camera and held a red filter over the lens. I repeated with a green filter, then a blue filter. Here are the results. Each has been slightly adjusted to generally match the exposures.

The red filter transmits red light. Any areas that are red on the quilt will show as a lighter, more exposed, shade of gray. Blue areas on the quilt should show as a dark shade of gray. The green filter will block red and blue colors from the quilt. Note the dark borders around the central medallion in the second image.

Red filtered

Red filtered

Green filtered

Green filtered

Blue filtered

Blue filtered

The next step was to open each of the 3 filtered images in the ImageJ software and have it merge them into a composite. I think it turned out quite well when compared with the original quilt image. It could use a little more green.

Composite of all 3

Composite of all 3



37 thoughts on “Astro-Images | Making Color Images

  1. Me too. This is so neat, Jim. Sometimes I get frustrated with the process of getting a digital image to approximate the appearance of my paint on canvas, but really it is kind of fun to play with the variables. I love the idea of you taking pictures of the stars.

  2. Some of the color images from distant nebulae are mind-boggling.They look so cohesive and orderly, but I imagine that is not the case. Given the ever-expanding size of the universe, I would think there is something new to discover regularly…maybe even daily. We are such tiny, insignificant bags of dust.

    • It is astounding how big and far apart things are. In some ways things are cohesive and orderly. The sequence of events from the time of moments after the Big Bang are pretty well known.

      This old bag of dust needs to get outside for a walk in the nice weather. See you later.

      • Nothing personal, Jim. 🙂 Whether one believes in scripture or not (I don’t), it is true where we come from and where we go….although I am impatient and will be cremated…too bad blowflies and carrion beetles.

        I hope you had a nice walk.

      • I took nothing personal there. My sense of order and cohesive is from a cosmological and natural viewpoint. I didn’t mean any scriptural sense.

        I’m going with cremation, too. 🙂

        We did have a nice walk of about 3 miles…third day in row of that distance. It’s like we are in training.

  3. What I wonder is whether the CCD devices are exclusive to the types of cameras being used. You cannot use them with a Canon or Nikon, can you? It’s exclusive for astronomical use, right?

    • The CCD devices are not able to be fitted to a consumer type camera. For one, they tend to be much bigger in area. The pixels are also closer together and need to be cooled. If not cooled, they give false readings.

      Their technology advances do get carried over into the cameras we use. That is a good thing.

  4. It’s similar to the notion of the B&W layer PS uses, or the RGB color model where an additive color model in which red, green, and blue light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three additive primary colors, red, green, and blue. Isn’t?

  5. Even before I got to the part of your post about Melanie’s quilt, I was reminded that in the 1800s, when black and white photography itself was still new, some people were already experimenting with combining black and white photographs taken with red, blue, and green filters to make real-color images. Apparently the first to do so was the physicist James Clerk Maxwell, as noted in the Colour Analysis section of the article at

    In searching for early examples of RGB photography, I came across a reference to Russian photographer Sergeii Prokudin-Gorskii, and a vivid example of one of his pictures:

    • Good for James. He was into everything. What a clever fellow.

      I sent a comment to the author of the 2nd linked article. The color rendition was great.

      Thanks for the links.

  6. […] With the recent passage of Comet Lovejoy, I renewed my interest in how colored astronomical images are made. Lovejoy has been imaged and displayed on this site for several weeks. The typical process involves making at least three images in greyscale through filters in the red, green, and blue parts of the electromagnetic spectrum. Suitable image processing software can then combine those three images into one with the appearance of full color. More details of the process I used can be found in this recent post. […]

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