The detectors we use on Chandra are different from detectors on optical telescopes. Most of the Chandra images are taken with what's known as a Charged Coupled Device (CCD). The CCD is the type of detector that's in the camera in your cell phone, or in your digital camera.
But in X-rays, CCDs have the ability to provide information about how energetic the X-ray was that was recorded. Optical light cameras use things that are sensitive to red, green and blue and combine them to make a color image. In X-rays that's not what we do. We have something that's sensitive to the whole X-ray range that we can collect with Chandra. For each one of the X-rays that we record, we have a measurement of how much energy is deposited in the CCD. The energy that is deposited is directly related to how much energy the X-ray itself had.
To make color images, we put together a similar mapping to what one would get in the visual band by saying, low energies in the optical band correspond to the red part of the spectrum, medium energies correspond to the green part of the spectrum and high energies are blue.
We take the low energy X-rays in an arbitrary defined band. From energy "x" to energy "y", we'll consider it to be low and we make those red on the picture. Then we pick an arbitrary range in the middle and we make those green, and finally we take another range in the higher end and make it blue. Scientists don't assign the same range for every object Chandra images. Typically, the energy ranges are chosen to emphasize particular processes.
So for example, in supernova remnant G292.0+1.8, we've chosen colors that emphasize emission from particular elements. Other times one wants to be able to just differentiate between low, medium and high energy (check out Westerlund 2).
But the amount of low energy radiation from some objects might be completely dominant and there might be not very much high-energy radiation which means the blue part of your picture would basically disappear. So sometimes, you need to shift the boundaries down a little bit in energy so that you collect enough X-rays in the high energy part of the picture to actually get enough blue ink on the page to make a picture.
Also keep in mind that for some of the images we don't put different X-ray energy bands into different colors. Instead we do things like put a radio image in red, an infrared image in green, and an X-ray image in blue (see M82). This gives us a really nice way of connecting the structure of objects as we see them in these completely different emission bands.
-Excerpted from Dr. Pat Slane's conversation with the NASA museum alliance in October 2007. Edited by Megan Watzke (CXC).