One very common operation with spacecraft images is to make a color image from individual frames captured through different filters. To make approximate true color images, an image of the same target needs to be captured through red, green, and blue filters at about the same time. False color images can be made from any combination of three filters.
If you'd like to try this at home, you will need:
A copy of Adobe Photoshop. The screen caps in this tutorial are from Photoshop CS3, but earlier versions will also work. (Note: making RGB images is possible with many off-the-shelf, shareware, or freeware image processing packages; this tutorial presently covers only Photoshop.)
Consider these three images taken from the Cassini raw images website. They were taken by Cassini on June 15, 2007 and show the moon Rhea against Saturn's globe. From left to right, they were taken through red, green, and blue filters. These images are courtesy of NASA, JPL, and the Space Science Institute.
Click to enlarge > This image was taken from 1,197,115 kilometers away, using the RED and CL2 filters.
Click to enlarge > This image was taken from 1,197,278 kilometers away, using the CL1 and GRN filters.
Click to enlarge > This image was taken from 1,197,277 kilometers away, using the BL1 and CL2 filters.
Preliminary Note about Image Credits
All raw images posted on NASA mission websites are in the public domain. However, you should always give credit to the appropriate institutions for taking the original images. Each mission's website will offer guidance on how images should be credited.
(For clarity, from here forward these images will be shown cropped more tightly around Rhea.)
This tutorial will show you how to convert three grayscale images into one color image. However, if you go straight to that step, you will find that because Cassini is not a fixed platform but instead a moving orbiter, Rhea shifts from frame to frame. If you do not align the frames before merging them, you will produce an image that looks like this, where each sharp edge is fringed with color:
Click to enlarge > If images captured through different filters are not aligned, color fringes will appear around sharp edges in the image. Credit: NASA / JPL / SSI / Emily Lakdawalla
Aligning the Frames
Begin by opening the three different-filter images of Rhea.
Paste the three frames into layers on one image.
Begin with the Blue-filter image.
Go to the Green-filter image, select all (Ctrl-A or Select > All), copy (Ctrl-C or Edit > Copy), return to the Blue-filter image, and paste (Ctrl-V or Edit > Paste).
Repeat the step with the Red-filter image.
You should now have one image with three layers. The Blue-filter image is on the bottom as the background layer, the Green-filter image is in the middle as Layer 1, and the Red-filter image is on top as Layer 2.
Now would be a good time to save (Ctrl-S or File > Save).
Click to enlarge > The three images have now been stacked as layers in a single Photoshop document. In this example, the Blue filter image is the Background layer, the Green filter image is Layer 1, and the Red filter image is Layer 2. Credit: NASA / JPL / SSI / Emily Lakdawalla
Increase the size of the image
Go to Image > Image Size... and make the image 200% or 300% its original size. This step will permit you to make finer adjustments to the alignment of the frames.
Align the green-filter image to the blue-filter image
In the Layers palette, turn off the top layer (Layer 2) by clicking the eye icon next to it.
Make the middle layer (Layer 1) active by clicking the text "Layer 1."
Set the blending mode for this layer to Difference by using the drop-down menu in the Layers palette. This blending mode subtracts one of the layers from the other. Since the two layers are very similar the image will now appear mostly black, because the two layers will nearly cancel each other out. However, where they are poorly aligned, there will be white fringes.
Click to enlarge > With one layer set to the Difference blending mode, that layer's pixel values are subtracted from the ones below it. The result is an image that is nearly black (because the two images are largely similar), but there are bright fringes because the two layers are not perfectly aligned. Credit: NASA / JPL / SSI / Emily Lakdawalla
Using the Move tool (), move Layer 1 around until the white fringes disappear (or are minimized). If the necessary adjustments are small, it may be easiest to use the arrow keys on your keyboard to nudge the layer back and forth.
Set the blending mode back to Normal.
Align the red-filter image to the blue-filter image
In the Layers palette, turn off Layer 1 and turn on Layer 2.
Repeat steps 3b through 3e for Layer 2.
Crop the image so that every layer reaches the edge of the canvas
Because you moved layers around, there will be "blank" or "transparent" areas at the extreme edges of some layers. These transparent areas can cause problems later when you paste the layers into a new document. You want to crop these transparent areas away from the image. You can do this either using the Crop tool () or by shrinking the size of the canvas (Image > Canvas Size…)
Now would be a good time to save.
Merging the aligned frames into an RGB image
Make the background layer (the blue-filter image) the active layer by clicking on its name in the Layers palette.
Copy the layer (select all and copy).
Create a new, blank RGB image (Ctrl-N or File > New; Photoshop will automatically fill in the Width and Height of your copied image; select "RGB Color" for the Color Mode.)
Activate the Channels palette by clicking the Channels tab next to the Layers tab.
Click on the Blue channel.
Paste the layer (Ctrl-V or Edit > Paste).
The blue channel now contains the blue-filtered Cassini image. You may notice that the RGB image in the Channels palette looks yellow.
Click to enlarge > Tutorial: RGB images: channels The new color image now has one of its channels filled in, the blue channel. Because the red and green channels have not been filled in yet, the resulting RGB image appears yellow. (The red and green channels have all-white, or maximum-value pixels, and red and green light mixed together make yellow.) Credit: NASA / JPL / SSI / Emily Lakdawalla
Activate the Layers palette by clicking the Layers tab.
Return to the image containing your aligned layers. Make Layer 2 (the green-filter image) active, select all and copy.
Return to the new color image, activate the Channels palette, select the Green channel, and paste. The RGB channel will now look red.
Repeat steps 8-10 for the last layer, the red-filter image, pasting it into the Red channel.
Make the RGB channel active. Your image should now look like this, with a yellowish Saturn and a grayish Rhea.
Save your artwork!
Finally, feel free to play around with the image's brightness, contrast, hue, and saturation using the various controls available to you in the Image menu.
Click to enlarge > Now that the layers have been properly aligned, the resulting color image of Rhea has no annoying color fringes. Credit: NASA / JPL / SSI / Emily Lakdawalla