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 Spaceweather.com are in color. The comet appears as a faint blue-green object like this fine example.
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.
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.
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.
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.
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.
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.