The color images in this calendar were produced with a black-and-white CCD (Charge-Coupled-Device) camera. For each image, three separate exposures were obtained through filters that approximate the primary colors of red, green and blue. They were then combined to produce the color image.
The images fall into two broad categories: "extended" color and "false" color. Extended color images were obtained through broad band filters, with red corresponding to near-infrared, along with blue and green.
In many cases (such as when luminous gas provides the light we see), false color images can reveal much about the physical nature of the object. For these, the red frame uses a narrow filter centered on the wavelength corresponding to hydrogen emission. The green frame shows the light of ionized oxygen, and the blue is either another line of hydrogen, or the broader blue filter.