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. The camera CCD is able to capture color images using red, green, and blue light filters. 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 Spaceweather.com are in color. The comet appears as a faint blue-green object.

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 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 requested three images from the Iowa Robotic Telescope in Arizona operated by the University of Iowa Physics and Astronomy Dept. for research and education. It operates with a greyscale CCD. There were options in the request form for 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.

Below are the three images after adjusting the minimum and maximum values of the range of exposure for each. The telescope tracked the comet instead of the stars. Notice the short star trails resulting from the 120 sec exposures.

Red filter used

Green filter used

Blue filter used

Desktop software allowed the three images to be opened and merged into a colot composite. 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.

Composite of 3 RGB images

Greyscale filtered images can be downloaded free from the Hubble Legacy Archive site or another major telescope sites. The images are publicly available…our tax dollars at work. Images such as this one were made using the technique described above.

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

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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 show as a lighter, more exposed, shade of gray. Blue areas on the quilt show as a dark shade of gray. The green filter blocks red and blue colors from the quilt. Note the dark borders around the central medallion in the second image when green was used.

Red filtered

Green filtered

Blue filtered

The next step opened each of the 3 filtered images in the software and merged them into a color composite. Compared with the original quilt image, it could use a little more green.

Composite of all 3