Most astronomical CCD cameras create color images by combining black and white images taken through red, green, and blue filters. In the Basics of Taking CCD Images section we gave the example of the Trifid Nebula, M20, imaged using this technique. We ended up with three image files, one for each color. After processing these images using the previous techniques of calibration and stacking, we are ready to combine them to create a full color image. To combine the images they must be aligned with each other. Some software packages allow you to align the images first and then combine them, while others combine the images first and then allow you to shift the images as necessary to align them. CCDOPS, included with the popular CCD cameras from SBIG is in the latter category so we will use that method here. The basic ideas are the same for any program, and other color-combining techniques are covered under Advanced Image Processing. See the Software Instructions section for specific details on color combining with CCDOPS, MaxIm DL, and Photoshop.
The columns at right are for shifting each image to register it with the others during the alignment portion of the procedure. First, we must combine the images. We do this by clicking Do It in the window. This should combine the images into a color image. However, the images will not likely line up at this point and an image like the one below will be produced.
It can easily be seen from the image which direction each color component must be moved to line up with the others. For example, in the image above, the red component needs to be shifted to the right and down slightly in order to line up with the green component. The blue image must be shifted a down and a bit left to align with the green. This is accomplished by entering the appropriate numbers into the boxes in the RGB Combine window. It make take several attempts to align the images perfectly. In the end, for our example files the following adjustments were made:
The result is a full color image of the Trifid Nebula!